Wireless sensing system using sensing device with excitation element

ABSTRACT

At least some aspects of the present disclosure feature a mobile sensing system comprising a sensing device for measuring a thermal property of an object, comprising an RF circuit and an antenna electronically coupled to the RF circuit, a sensor electronically coupled to the RF circuit, and a thermal source thermally coupled to the sensor and electronically coupled to the RF circuit, a mobile device having a processor, an RF reader connected to or integrated with the mobile device, wherein the RF reader is configured to interrogate the sensing device; wherein the sensing device receives power when the RF reader interrogates the sensing device and provides at least a portion of the power to the thermal source.

CROSS REFERENCE OF RELATED APPLICATIONS

This application is a Continuation of U.S. application Ser. No.16/413,890, filed 16 May 2019, now granted U.S. Pat. No. 10,845,252 withan issue date of 24 Nov. 2020, which is a Divisional of U.S. applicationSer. No. 15/521,713, filed 25 Apr. 2017, which is a 371 ofPCT/US2015/058722, filed on 3 Nov. 2015, which claims the benefit ofU.S. Provisional Application No. 62/077,002, filed 7 Nov. 2014, thedisclosures of all of which are incorporated by reference herein intheir respective entireties.

TECHNICAL FIELD

The present disclosure relates to wireless sensing devices and systems.At least part of the present disclosure relates to wireless sensingdevices having excitation components. At least part of the presentdisclosure relates to wearable wireless sensors for measuring one ormore thermal properties.

SUMMARY

In one embodiment, a mobile sensing system comprises a sensing devicefor measuring a thermal property of an object, comprising an RF circuitand an antenna electronically coupled to the RF circuit, a sensorelectronically coupled to the RF circuit, and a thermal source thermallycoupled to the sensor and electronically coupled to the RF circuit, amobile device having a processor, an RF reader connected to orintegrated with the mobile device, wherein the RF reader is configuredto interrogate the sensing device; wherein the sensing device receivespower when the RF reader interrogates the sensing device and provides atleast a portion of the power to the thermal source, wherein the sensoris configured to generate a series of sensing signals associated withtemperature when the thermal source is powered, wherein the sensingdevice is configured to transmit a series of data signals associatedwith the series of sensing signals via the antenna; and wherein the RFreader receives the series of data signals and the processor isconfigured to determine the thermal property of the object using atleast some of the series of data signals.

In one embodiment, a wireless sensing system comprises an RF sensor tagconfigured to evaluate a physical characteristic of an object,comprising an RF circuit and an antenna electronically coupled to the RFcircuit, a sensor electronically coupled to the RF circuit, and anexcitation device configured to be activated and generate an excitationsignal to change a condition, wherein the sensor generates a sensingsignal associated with the physical characteristic of the object thatvaries in response to the changed condition when the excitation deviceis activated, wherein the RF sensor tag is configured to transmit a datasignal associated with the sensing signal via the antenna; and an RFreader configured to transmit an interrogation power to the RF sensortag to activate the excitation device and receive the data signal,wherein the RF reader is further configured to adjust the interrogationpower based on the data signal.

In one embodiment, a wireless sensing system comprises a plurality ofsensing devices, each sensing device comprising a wireless transceiverand an antenna electronically coupled to the wireless transceiver, asensor electronically coupled to the wireless transceiver, and anexcitation device configured to generate an excitation signal, whereinthe sensor produces a sensing signal when the excitation signal isgenerated, wherein the sensing device is configured to transmit a datasignal associated with the sensing signal via the antenna; and a readerconfigured to transmit activation signals to the plurality of sensingdevices to activate the excitation devices and receive data signals fromthe plurality of sensing devices, wherein at least some of the pluralityof sensing devices are disposed at locations different from each other,wherein the reader is further configured to coordinate a transmissionorder of activation signals to the plurality of sensing devices in apattern related to the location of sensing devices.

BRIEF DESCRIPTION OF DRAWINGS

The accompanying drawings are incorporated in and constitute a part ofthis specification and, together with the description, explain theadvantages and principles of the invention. In the drawings,

FIG. 1A illustrates a block diagram of one embodiment of a wirelesssensing device;

FIG. 1B illustrates a block diagram of another embodiment of a wirelesssensing device;

FIG. 1C illustrates a block diagram of yet another example of wirelesssensing device;

FIG. 2A is a simplified schematic of one embodiment of an RF sensor tag;FIG. 2B is a cross sectional view of the wireless sensing deviceillustrated in FIG. 2A at arrow 2B;

FIG. 2C is a simplified schematic of one embodiment of a wirelesssensing device; FIG. 2D is a simplified schematic of one embodiment of apower measurement circuit;

FIGS. 3A-3C illustrate simplified schematics of some embodiments ofwireless sensing device with multiple sensors; and FIG. 3D is across-sectional view of the wireless sensing device illustrated in FIG.3A;

FIGS. 4A and 4B illustrate simplified schematics of some embodiments ofa wireless sensing device with multiple sensors and/or multiple RFdevices;

FIG. 5 illustrates a simplified schematic of one embodiment of awireless sensing device with multiple sensors and a single antenna;

FIG. 6A illustrates one embodiment of a mobile sensing system;

FIG. 6B illustrates a block diagram illustrating an example of a mobilesensing system including a mobile device and a wireless sensing device;

FIG. 6C illustrates one embodiment of a wireless sensing system;

FIG. 6D illustrates one embodiment of a wireless sensing system havingmore than one wireless sensing devices;

FIG. 7A illustrates an example flowchart for the operation of oneembodiment of a wireless sensing device and/or system;

FIG. 7B illustrates an example flowchart for the operation of oneembodiment of a wireless sensing device or system having two sensors;

FIG. 7C illustrates an example flowchart to determine a hydration level;

FIG. 8A illustrates a graph of an example of magnetic coupling v.frequency;

FIG. 8B illustrates an example of temperature-time profile;

FIG. 8C is a concept example that indicates the usefulness ofcontrolling the power delivered to a thermal source;

FIG. 9A illustrates one embodiment of hydration sensing system;

FIG. 9B shows a schematic temperature-time profile of a thermal sourcebefore, during, and after a constant input power;

FIG. 9C illustrates a cross-section view of some of the components forone embodiment of a wireless sensing device for measuring a liquidlevel;

FIG. 9D illustrates an schematic diagram of one embodiment of a wirelesssensing device for measuring a liquid level;

FIG. 10 illustrates a simplified schematic of one example of a wirelesssensing device;

FIG. 11 is a picture of an example of a wireless sensing device with twointegrated circuits; and

FIG. 12 illustrates an example graph of temperature verse time.

In the drawings, like reference numerals indicate like elements. Whilethe above-identified drawing, which may not be drawn to scale, setsforth various embodiments of the present disclosure, other embodimentsare also contemplated, as noted in the Detailed Description. In allcases, this disclosure describes the presently disclosed disclosure byway of representation of exemplary embodiments and not by expresslimitations. It should be understood that numerous other modificationsand embodiments can be devised by those skilled in the art, which fallwithin the scope and spirit of this disclosure.

DETAILED DESCRIPTION

Unless otherwise indicated, all numbers expressing feature sizes,amounts, and physical properties used in the specification and claimsare to be understood as being modified in all instances by the term“about.” Accordingly, unless indicated to the contrary, the numericalparameters set forth in the foregoing specification and attached claimsare approximations that can vary depending upon the desired propertiessought to be obtained by those skilled in the art utilizing theteachings disclosed herein. The use of numerical ranges by endpointsincludes all numbers within that range (e.g. 1 to 5 includes 1, 1.5, 2,2.75, 3, 3.80, 4, and 5) and any range within that range.

As used in this specification and the appended claims, the singularforms “a,” “an,” and “the” encompass embodiments having pluralreferents, unless the content clearly dictates otherwise. As used inthis specification and the appended claims, the term “or” is generallyemployed in its sense including “and/or” unless the content clearlydictates otherwise.

Spatially related terms, including but not limited to, “lower,” “upper,”“beneath,” “below,” “above,” and “on top,” if used herein, are utilizedfor ease of description to describe spatial relationships of anelement(s) to another. Such spatially related terms encompass differentorientations of the device in use or operation in addition to theparticular orientations depicted in the figures and described herein.For example, if an object depicted in the figures is turned over orflipped over, portions previously described as below or beneath otherelements would then be above those other elements.

As used herein, when an element, component or layer for example isdescribed as forming a “coincident interface” with, or being “on”“connected to,” “coupled with” or “in contact with” another element,component or layer, it can be directly on, directly connected to,directly coupled with, in direct contact with, or intervening elements,components or layers may be on, connected, coupled or in contact withthe particular element, component or layer, for example. When anelement, component or layer for example is referred to as being“directly on,” “directly connected to,” “directly coupled with,” or“directly in contact with” another element, there are no interveningelements, components or layers for example. As used herein,“electronically coupled” and “electrically coupled” are usedinterchangeably.

As used herein, layers, components, or elements may be described asbeing adjacent one another. Layers, components, or elements can beadjacent one another by being in direct contact, by being connectedthrough one or more other components, or by being held next to oneanother or attached to one another. Layers, components, or elements thatare in direct contact may be described as being immediately adjacent.

Some aspects of the present disclosure are directed to the developmentof sensors built upon radio frequency (RF) technology, which is anattractive approach based on the ability for wireless data and powertransfer. As used herein, RF is used to refer to a broad class ofwireless communication interface that can provide communications andpower, including far field communication and near field communication(NFC), which may utilize a specific communication protocol. Sensorsbased on RF technology enable beneficial system attributes, such aswireless readout, passive (battery-free) sensor operation, unique sensoridentification, compatibility with the human body, and onboardmicro-processing capabilities. In addition, the growing prevalence ofNFC enabled smart devices, such as smart phones, tablets, and smartwatches, allows readout of RF based sensors without the need for aspecialized reader. NFC includes but is not limited to the set ofstandard protocols defined by the NFC Forum industry association.

At least some aspects of the present disclosure feature wireless sensingdevices for measuring thermal properties that can transmit sensorsignals via a wireless interface. A wireless interface includes farfield communications and NFC. In some embodiments, the wireless sensingdevice use NFC as communication interface. At least some aspects of thepresent disclosure feature wireless sensing devices having a thermalspreader, also referred to as thermal region or heat spreading layer,where the thermal spreader is formed by solid or liquid materials thathave relatively high thermal conductivity in comparison to the thermalconductivity of gas, and a thermal source and a sensor disposed in thethermal spreader to allow measurement of one or more thermal properties.In some cases, the wireless sensing device includes an energy harvestingdevice to receive and convert power and supply power to at least some ofthe other components of the wireless sensing devices.

In some embodiments, a wireless sensing device has a single antenna withtwo or more sensors and excitation devices, where each sensor is coupledto an RF device having a unique identifier and each sensor is coupled toan excitation device. In such embodiments, the wireless sensing devicecan provide sensor signals from the spatially dispersed sensors to acomputing device to determine physical properties based on spatialinformation and sensor signals.

In some embodiments, a wireless sensing device has thermally isolatedregions, where each region includes a thermal sensor and optionalthermal source, the measurement of the thermal sensors can be used todetermine thermal properties of an object of interest. In oneembodiment, the wireless sensing device has one thermal region Acomprising materials with high thermal conductivity and a region B thatis thermally isolated from the region A, where the region A is inthermal contact with an object of interest. In such embodiment, thethermal property of the object can be determined based on differentialsensor signals from sensor A disposed proximate to region A and sensor Bdisposed proximate to region B.

At least some aspects of the present disclosure are directed to awireless sensing system having an RF reader and a wireless sensingdevice of any configuration described herein. In some embodiments, thewireless sensing system includes multiple sensors that are spatiallyseparated and receives sensing signals from these sensors. In some casesof such embodiments, the wireless sensing system can establish an arrayor a map of sensing signals and thereby evaluate physical properties ofa material or object at various parts corresponding to sensor positions.In some other embodiments, the wireless sensing system receives sensingsignals that are temporally separated, from a wireless sensing device tomeasure a physical property of an object. In some cases of suchembodiments, the wireless sensing system can establish a profile ofsensing signal changes over time and thereby determine physical propertyof the object based on the profile. For example, the wireless sensingsystem can establish a temperature-time profile with one or more thermalsensors and determine water content within the material or object andfurther evaluate a hydration or moisture level of the material orobject.

Thermal properties of a material or an object include, for example,thermal conductivity, thermal conductance, specific heat capacity, heatcapacity, thermal diffusivity, or the like. Thermal conductivity isintrinsic temperature difference in response to an applied heat fluxthrough a material, with typical units of power per length-temperature,such as watts per meter-Kelvin. Thermal conductance takes into accountcross-sectional area of heat flux and material thickness, with typicalunits of power per temperature, such as watts per Kelvin. Specific heatcapacity is intrinsic temperature rise in response to heat energy, withtypical units of energy per mass-temperature, such as joules perkilogram-Kelvin. Heat capacity takes into account mass of material, withtypical units of energy per temperature, such as joules per Kelvin.Thermal diffusivity is the ratio of thermal conductivity to the productof mass density and specific heat capacity and indicates how quickly amaterial would reach a temperature similar to its surroundingenvironment, with typical units of area per time, such as square metersper second.

A material or an object can be a composite, where thermal properties ofa composite refer to effective or average thermal properties for thecomposite. Some composites have a dispersed phase in a dispersionmedium, which are generally known as solutions, as colloids, or assuspensions depending on the length scale involved. The composite can beone-phase or mixed-phase, containing one or more of solids, liquids, orgases.

In some embodiments, thermal properties of composites whose dispersionmedium is not a gas (i.e., non-aerosol composites) are measured and/orcalculated. The non-aerosol composites include, for example, foams(i.e., gas dispersed in solid or liquid), emulsions (i.e., liquiddispersed in liquid or in solid), or sols (i.e., solid dispersed inliquid or in solid). The non-aerosol composites may also include, forexample, a heterogeneous mixture of solids, liquids, or gases that aredispersed in a solid or liquid matrix. For example, the effectivethermal conductivity of an acrylate adhesive having dispersed particlesof aluminum oxide is based on the thermal conductivity of each material,the mixing fraction, and other properties such as particle shape.

FIG. 1A illustrates a block diagram of one embodiment of a wirelesssensing device 100A, which can be used to measure a thermal property ofan object. As described herein, a wireless sensing device is typicallyin an assembly. In the embodiment illustrated, the wireless sensingdevice 100A includes a substrate 110, a control circuit 120 disposed onthe substrate 110, a transceiver 130 electronically coupled to thecontrol circuit, an antenna 135 electronically coupled to thetransceiver and disposed on the substrate 110, an optional energyharvesting device 140 disposed on the substrate, an optional thermalsource 150 and a sensor 160. In some cases, the energy harvesting device140 is electronically coupled to the antenna 135. The antenna 135 isconfigured to transmit signals when an RF reader interrogates thewireless sensing device 100A, for example.

In some configurations, the wireless sensing device 100A has an optionalthermal spreader 115, which includes solid, liquid, or compositematerial and has a desired or known thermal property. In some cases, thethermal conductivity of the thermal spreader 115 is higher than thethermal conductivity of the substrate 110. In some other cases, thesubstrate is the thermal spreader that is configured to be in thermalcontact with the object when the wireless sensing device is in use formeasuring the thermal property of the object. For example, the thermalspreader 115 includes a polymer film or an adhesive layer. In someimplementations, the thermal spreader 115 has a thermal conductivitygreater than or equal to 0.1 watts per meter-Kelvin. The thermalspreader 115 may include metallic fillers, such as aluminum, or ceramicfillers, such as boron nitride. In some cases, fillers used in thethermal spreader are to obtain a desired thermal conductivity. In someembodiments, the wireless sensing device 100A may improve accuracy ofmeasurement results by providing generally uniform thermal flux throughthe thermal spreader 115. In some cases, the wireless sensing device100A may use the thermal property of the thermal spreader 115 indetermining the thermal property of the object.

Thermal flux or heat flux is the transfer of thermal energy through amedium by conduction (phonons), convection (fluid flow), or radiation(photons). The thermal flux of primary interest is that the thermal fluxmoves to and from a thermal source by conduction, and where the thermalflux is spread to or from a surface of a region or substrate.

The substrate 110 can be flexible or rigid. In some embodiments, thesubstrate 110 is stretchable. In some embodiments, the substrate 110includes polyurethane. In some embodiments, the substrate 110 is apolymeric film. Suitable polymer films include elastomeric polyurethane,co-polyester, or polyether block amide films.

The control circuit 120 can include one or more electronic componentsthat are electronically connected. The control circuit 120 can includepassive electronic components, for example, such as resistors,capacitors, inductors, transformers, diodes, and the like. The controlcircuit 120 can include active electronic components such astransistors, voltage or current sources, amplifiers, microprocessors,oscillators, analog-to-digital converters, digital-to-analog converters,phase-locked loops, and the like. In some cases, the control circuit 120may be formed into an integrated circuit or include an integratedcircuit. A microprocessor may be a state machine with relatively simpledigital logic to move among two or more states in a pre-defined manner,or a microcontroller comprised of an instruction set, digital processingblocks, memory, firmware, and peripherals such as clocks, memorycontrollers, and data converters. In some cases, the control circuit 120comprises a microprocessor and a memory storing a unique identifier. Insome embodiments, the control circuit 120, the transceiver 130, and theantenna 135 are components of a radio frequency identification (RFID)tag.

RFID tags on flexible and/or stretchable substrates are described inmore details in U.S. Patent Application No. 62/031,581, entitled “RFIDTag on Stretchable Substrate” and filed on Jul. 31, 2014, and U.S.Patent Application No. 62/031,603, entitled “RFID Tag on FlexibleSubstrate” and filed on Jul. 31, 2014, the entirety of which areincorporated herein by reference.

In some cases, the thermal source 150 is disposed proximate the thermalspreader 115 to generate thermal flux in the thermal spreader 115. Thethermal source 150 is electronically coupled to the energy harvestingdevice 140 and generates thermal flux in the thermal spreader 115. Insome embodiments, the sensor 160 is disposed in the thermal spreader 115and electronically coupled to the control circuit 120. The sensor 160 isconfigured to generate a sensor signal indicating a temperature andprovide the sensor signal to the control circuit 120. In some cases, thethermal source 150 and the sensor 160 are components of an integratedcircuit. In some implementations, the thermal source 150 and the sensor160 are a same resistive element.

The thermal source 150, also referred to as heating element, maygenerate heat by Joule heating, for example, by passing current throughany electrical component that has a non-zero electrical resistance. Forexample, the thermal source 150 can be a resistor electronicallyconnected to a source of current, or indirectly such as a metallic ormagnetic material coupled to a changing magnetic field to produceelectrical current by magnetic induction.

In some cases, the thermal source 150 can be a thermoelectric deviceoperating based on the Peltier effect, such as a thermoelectric heateror cooler containing one or more junctions of p-type and n-typethermoelectric materials, typically wired electronically in series.Depending on the polarity of the electrical current, one portion of thethermoelectric device will increase in temperature and another portionwill decrease in temperature, so the thermoelectric device may be usedfor heating and/or cooling. Such thermoelectric thermal sources may alsohave thermal contributions from Joule heating due to the non-zeroelectrical resistance of the elements.

In some cases, the thermal source 150 can be based on opticalabsorption, from an intentional source or from an ambient source ofoptical energy. In some other cases, the thermal source can include aparasitic element or otherwise unintentional source of heating orcooling. In some cases, the thermal source 150 is a dedicated componentin the wireless sensing device 100A. In some other cases, the thermalsource 150 includes one or more electronic components that are inoperation in the wireless sensing device. For example, the thermalsource 150 may include a transceiver element that generates additionalheating during operation. As another example, the thermal source 150includes a microprocessor element that generates heat during operation.

In some embodiments, the thermal source 150 is disposed proximate to thethermal spreader 115. In some cases, the thermal spreader 115 has aknown thermal property, which can be used to determine one or morethermal properties of the object. In some implementations, the thermalsource includes a high conductivity component. In some embodiments, thethermal source 150 and/or thermal spreader 115 is in thermal contactwith the object or material of interest. Thermal contact is defined atan interface of two materials, where non-infinite thermal contactconductance results in a temperature difference across the interfacewhen heat flux moves across the interface. The interface generallyconsists of a mixed-phase region similar to some of the compositematerials described. In some embodiments, the interface may comprisesolid regions with some roughness or within the solid regions, wherefluid regions within that roughness. Fluid regions can include liquid,gas, or a mixture. In some embodiments, the interface may have solid orliquid regions with gas regions in one or more voids or at a surface.Maintaining good thermal contact or thermal coupling typically involveslimiting the fraction of fluid regions especially those containing gas.Thermal interface materials are typically used for this purpose, such aselastomeric pads, adhesive tapes, greases, or the like. Effectivecontact thermal conductance is the inverse of temperature differenceacross a contact interface area for a given heat flux, having typicalunits of watts per square meter per Kelvin. Effective contact thermalconductance may be additionally scaled by an effective thickness of thecontact region to obtain an effective contact thermal conductivity,having typical units of watts per meter per Kelvin.

In some embodiments, the sensor 160, also referred to as sensingelement, may be a thermal sensor that has measurable changes inelectrical property, optical property, acoustic property, or the like,in response to temperature changes. In some cases, electrical thermalsensors can have a response to temperature changes in electricalvoltage, current, or resistance. A resistive thermal sensor has itselectrical resistance dependent on temperature; typical metals areresistive thermal devices where resistance increases with temperature ina relatively linear relationship. A thermistor typically has aresistance that depends on electrical current and non-linear resistancechanges in response to temperature changes. In some implementations,electrical thermal sensors may operate based on the Seebeck effect toconvert a temperature difference into an electrical voltage, such as athermocouple or thermopile.

An optical temperature sensor includes an optical transducer thatreceives electromagnetic radiation from objects out of thermalequilibrium with their environment, where the transducer temperaturechanges as it absorbs and emits radiation, for example, bolometers,microbolometers, pyroelectric detectors, or the like. These sensorscombine optical and electrical aspects, where incident and reflectedradiation is measured and converted to an electrical response when thetransducer is heated or cooled by the radiation.

An acoustic temperature sensor relies on the temperature-induced changein propagation of mechanical waves through a bulk material or along thesurface of a material. A sensor to measure temperature can comprisemultilayer structures that deform in response to temperature based ondiffering thermal expansion properties of the layers. The deformationcan be transduced electronically, such as a deformed beam that completesan electromechanical switch, or transduced as a visible indicator bymeans of a dial or other element.

In some cases, the energy harvesting device 140 comprises a bridgerectifier, a rectifier, a diode or transistor rectifier, and may includea voltage or current regulator. In some implementations, non-rectifiedelectrical power may be provided to the thermal source (e.g., resistor),while the remainder of the electronic circuitry typically operates onrectified power. An energy harvesting device can receive power from anintentional radiation source or from an unintentional or ambient source.Intentional radiation sources may include, for example, an RF reader.For example, depending on the configuration of its electronics,antennas, and frequency ranges of operation, an RF reader can produce anear-field electrical or magnetic field that stores energy for couplinginto one or more target devices, or it can produce a far-field radiationpattern of traveling electromagnetic waves, or a combination thereof. Insome cases, the magnetic field may couple to the antenna 135 and theenergy harvesting device 140 to induce an electrical current in thewireless sensing device from the magnetic field.

The energy harvesting device 140 may also be coupled to an unintentionalor ambient source, for example, such as an optical source, inertialvibration source, or temperature gradient source, or the like. Anoptical source can be, for example sunlight, artificial lighting, or thelike. In such examples, the energy harvesting device 140 can includephotovoltaic cells to convert optical energy to electrical energy. Aninertial or vibration source of energy can be, for example, a motor, amoving transportation vehicle (e.g., automobile, train, airplane, etc.),wind, or the like. It can also be a biological source such as a human inmotion. In such examples, the energy harvesting device can include apiezoelectric device that converts mechanical energy to electricalenergy. The energy harvesting device 140 can obtain electrical energyfrom a temperature gradient. For example, the energy harvesting devicecan include a thermoelectric device operating based on Seebeck effectthat converts the temperature gradient and heat flow resulting from theskin of a mammal or from the outside of a pipe containing a processfluid into electrical energy.

In some embodiments, the thermal source 150 is regulated. The thermalsource 150 can be regulated by the processing components of the controlcircuit 120, by interaction with an external device through thetransceiver 130, the energy harvesting device 140, or by a combinationthereof. In some cases, the control circuit 120 regulates the thermalsource 150. In some cases, the control circuit 120 regulates the thermalsource 150 based on sensor signals.

In some embodiments, the power delivered to the thermal source 150 canbe modulated by a controller within the control circuit 120, forexample, by changing the amount of electrical power delivered to thethermal source 150. In some embodiments, the control circuit 120regulates the electrical power in order to maintain constant temperatureor maintain a desired rate of change in temperature in response to thesensing signal indicative of temperature provided by the sensor 160,which is a closed-loop control based on temperature. In someembodiments, the control circuit 120 regulates the electrical power intothe thermal source in order to maintain constant power or a desired rateof change in power supplied to the thermal source 150, which is aclosed-loop control based on power. In some other embodiments, thethermal source 150 receives a known but uncontrolled current, voltage,or power, and the known value is used in later computation step(s) toaccount for variation in power which is an open-loop control. In yetsome other embodiments, an open-loop type of control may be implementedwith contingent constraints, such as a maximum or minimum value of asensing signal obtained from sensor 160 and used by the control circuit120 to adjust the thermal source 150 if the sensing signal is outsidethe maximum or minimum value.

In some embodiments, the thermal source 150 can be regulated byperforming the measurement during a time period when a dominantparasitic/unintended heating element is operating. Such regulation canbe done directly through an intentional increase in the operating loadof the parasitic/unintended heating element. For example, the controllercan instruct a transceiver to turn on and process otherwise nonsensicaldata in order to generate additional heat. In such example, the powersupplied to the transceiver can be monitored by the controller, with theprocessing load adjusted to maintain a constant power or desired rate ofchange in power.

In some embodiments, the wireless sensing device 100A may include anoptional energy storage device 170 that is disposed on the substrate andelectronically coupled to the energy harvesting device 140. Energystorage devices 170 may include capacitors or supercapacitors. Theenergy storage device 170 may store energy harvested from the energyharvesting device 140 for a short or long term period of time. Energystored in the energy storage device 170 may be used to provide power todesignated components of the wireless sensing device 100A including, butnot limited to, the control circuit 120, thermal source 150, and sensor160. When external energy is not available to the energy harvestingdevice 140, the wireless sensing device 100A can continue to operate onpower stored in the energy storage device 170. Also, energy stored inthe energy storage device 170 can be used to augment the power availablefrom the energy harvesting device 140, enabling higher poweravailability than the power available from the energy storage device 170or the energy harvesting device 140 alone.

In some embodiments, the energy harvesting device 140 provides power tothe thermal source 150, and possibly other components of the wirelesssensing device 100A, such as the sensor 160, the control circuit 120,and the transceiver 130. In some cases, the sensor 160 is configured togenerate a first sensor signal before the thermal source 150 isactivated by the energy harvesting device 140 and a second sensor signalafter the thermal source 150 is activated by the energy harvestingdevice 140. In such cases, the control circuit can determine a thermalproperty of the object based on the first and second sensor signals. Insome cases, the sensor 160 is configured to generate a first sensorsignal approximately concurrent with the activation of the thermalsource 150 by the energy harvesting device 140, and a second sensorsignal after the thermal source 150 is activated by the energyharvesting device 140.

The transceiver 130 can include a transmitter element and/or a receiverelement. A transmitter element includes one or more electromagnetic orelectroacoustic transducers, and electronic components to filter,amplify, and modulate one or more signals. A receiver element comprisesone or more electromagnetic or electroacoustic transducers that can beshared with those of the transmitter element via a switching means orcan be separate from those of the transmitter element, and electronicsto filter, amplify and demodulate one or more signals from the receivedenergy. An electromagnetic transducer can be an antenna, which can bedesigned to radiate electromagnetic fields from input electrical signalsand absorb electromagnetic fields into electrical signals, or can bedesigned to couple with stored energy in electromagnetic near fields, ora combination of both radiation and near-field coupling. Anelectromagnetic transducer can also be a light-emitting diode or otheroptical source, or a photodiode or other optical detector. Anelectroacoustic transducer can be a loudspeaker or other acousticsource, or a microphone or other acoustic detector. Electromagneticand/or electroacoustic transducers can be combined into a single elementthat is capable of bidirectional transduction from electrical signals toelectromagnetic or acoustic energy, and from electromagnetic or acousticenergy to electrical signals.

As an example, the transceiver 130 can be included in an integratedcircuit device, for example, NTAG213 from NXP Semiconductors (Eindhoven,the Netherlands). As another example, the transceiver 130 can be aninfrared transceiver element with a light-emitting diode, a photo diode,and accompanying electronics to implement optical communications via aninfrared protocol, for example, RPM841-H16 IrDA Infrared CommunicationModule from Rohm Semiconductor (Kyoto, Japan).

The antenna 135 can be a coil antenna designed for near-field couplingwith an RF reader. In some cases, the antenna 135 has a spiral form. Insome implementations, the antenna 135 comprises a plurality ofsubstantially concentric electrically conductive loops. In someconfigurations, the antenna has a length between first and second ends,the length being less than about 2 meters. In some cases, the antenna135 performs modulation and demodulation according to the standards, ISO14443A, ISO 15693, or other standard or proprietary communicationprotocols. The coil antenna can have an inductance based on its geometrythat produces a resonance with the capacitance of the electronicallyconnected components, generally referred to as RF components, forenhanced induced voltage for a given magnetic field strength near thefrequency of the RF reader. In some embodiments, the coil antenna mayhave an inductance based on its geometry that produces a first resonancewith a first capacitance of the RF components and a second resonancewith a second capacitance of the RF components, wherein the secondresonance is more closely matched with the frequency of the RF reader,coupling more energy into the wireless sensing device 100A due to theincreased induced voltage for a given reader magnetic field strengthwhen resonance frequency more closely matched with the RF readerfrequency. In some implementations, the RF components, which includecomponents of the transceiver 130 and/or control circuit 120, may beconfigured to contain a tunable or switchable capacitance to produce theat least two values of capacitance (i.e., the first capacitance, thesecond capacitance), or may contain circuitry for controlling anexternal variable capacitance, or may contain circuitry to allow one ormore external capacitance elements to be switched in or out of thecircuit.

FIG. 1B illustrates a block diagram of another embodiment of a wirelesssensing device 100B, which can be used to measure physical property ofan object. In the embodiment illustrated, the wireless sensing device100B includes a substrate 110, a control circuit 120, a transceiver 130electronically coupled to the control circuit, an antenna 135electronically coupled to the transceiver and disposed on the substrate110, an optional energy harvesting device 140 disposed on the substrate,a first excitation device 150B, a first sensor 160B, a second excitationdevice 155B, and a second sensor 165B. In some cases, the energyharvesting device 140 is electronically coupled to the antenna 135. Insome embodiments, the wireless sensing device 100B includes an optionalenergy storage device 170. In some cases, the energy storage device 170is electronically coupled to the energy harvesting device 140. In someembodiments, the wireless sensing device 100B includes a battery (notillustrated in FIG. 1B). Components with same labels can have same orsimilar configurations, compositions, functionality and/or relationshipsas the corresponding components in FIG. 1A.

The first excitation device 150B and the second excitation device 155Bcan include one or more of thermal excitation device, light excitationdevice, sound excitation device, vibrator, voltage source, currentsource, electromagnet, or the like. An excitation device can generate anexcitation signal and/or excitation signals during a period of time. Theexcitation signal can include, for example, a light signal, a voltagesignal, a vibration signal, a sound signal, heating or cooling signal,an electromagnetic signal, a current signal, or the like. The excitationdevices (150B, 155B) can initiate an excitation signal to change acondition, and the sensor (160B, 165B) can sense the physicalcharacteristics of the object that varies in response to the changedcondition and then determine one or more physical characteristics of theobject. As one example illustrated in FIG. 1A, the excitation device canbe a thermal source in thermal contact with the object, and the sensoris selected to measure temperature changes of the object. As anotherexample, the excitation device can be a vibration motor in contact withthe object, and the sensor can be an accelerometer.

In some cases, the excitation device (150B and/or 155B) can generateoptical energy, for example, such as light source, or the like; and thecorresponding sensor can include optical sensor, for example, such asphotodiode, photovoltaic sensor, or the like. In some cases, theexcitation device can include a motion source, for example, such asvibration motor, piezoelectric actuator, or the like; and thecorresponding sensor can include a motion sensor, for example, such aspiezoelectric sensor, accelerometer, or the like. In some other cases,the excitation device can include an acoustic source, for example, suchas microphone, piezoelectric transducer, or the like; and thecorresponding sensor can include an acoustic sensor, such as microphone,accelerometer, or the like. In yet some other cases, the excitationdevice can include an electrical source, for example, such as voltagesource, current source, or the like; the corresponding sensor caninclude an electrical sensor, for example, such as voltage sensor,current sensor, phase sensor, resistance sensor, or the like. In someimplementations, the wireless sensing device can include more than onetype of excitation devices, for example, both an optical source and amotion source, and/or more than one type of sensors, for example, bothan optical sensor and a motion sensor.

In some embodiments, the wireless sensing device 100B can include two ormore sensors spatially separated to measure physical properties atdifferent parts of the object. The sensor data can be used to, forexample, increase accuracy of measurement results, measure flow rate,detect anomalies in the object, or evaluate other properties of theobject. In some cases, the excitation device 150B and/or 155B can beregulated by the control circuit 120. In some cases, the excitationdevice 150B and/or 155B can be regulated by the control circuit 120based on sensor signal.

In some configurations, the wireless sensing device 100B has an optionalsensing region 115B and/or sensing region 117B, which comprisesmaterials suitable for a specific energy transfer. For example, thesensing region 115B and/or 117B includes a polymer film or an adhesivelayer suitable for thermal energy transfer. As another example, thesensing region 115B and/or 117B includes a reflective film suitable fordirecting light to the object.

In one embodiment, the excitation device 150B and 155B are thermalsources. The sensing regions 115B and 117B are thermally isolated fromeach other. The first thermal source 150B is disposed in the firstsensing region 115B and is electronically coupled to the energyharvesting device 140. The first sensor 160B is disposed in the firstsensing region 115B and electronically coupled to the control circuit120. The first sensor 160B is configured to generate a first sensorsignal associated with temperature. The second thermal source 155B isdisposed in the second sensing region and electronically coupled to theenergy harvesting device 140. The second sensor 165B is disposed in thesecond sensing region 117B and electronically coupled to the controlcircuit 120. The second sensor 165B is configured to generate a secondsensor signal associated with temperature. The control circuit 120 isconfigured to determine a thermal property of the object based on thefirst and second sensor signals. In some cases, the sensing region 117Bis in thermal contact with the object and the sensor 160B in a thermalisolated region 115B can provide baseline information to improve themeasurement accuracy of the wireless sensing device 100B.

In one embodiment, a thermal insulator is disposed between the sensingregions 115B and 117B. The thermal insulator can include, for example,foam, air gap, or the like. Foam may include, for example, any solidmaterial having voids, such as a polymer material with open-cell orclosed-cell voids, or a nonwoven polymer material. Thermal insulation isalso provided by geometry, for example by the ratio of separationdistance between regions to the cross-sectional area of the span betweenregions; a larger temperature difference is required to transfer a givenamount of heat when that ratio is larger.

In some cases, the first sensing region 115B and/or the second sensingregion 117B is on the substrate 110. In some other cases, the firstsensing region 115B and/or the second sensing region 117B is not on thesubstrate 110. In one embodiment illustrated in FIG. 1B, the controlcircuit 120 receives sensing data from both sensors 160B and 165B. Insome cases, the control circuit 120 includes a microprocessor todetermine the physical property of the object based on the datacollected by both sensors 160B and 165B. In some other cases, thecontrol circuit 120 transmits the sensor data via the transceiver 130for further processing.

FIG. 1C illustrates a block diagram of yet another example of wirelesssensing device 100C, which can be used to measure one or more physicalproperties of an object. In the embodiment illustrated, the wirelesssensing device 100C includes a substrate 110, a first control circuit120, a first transceiver 130 electronically coupled to the first controlcircuit 120, a second control circuit 125, a second transceiver 132electronically coupled to the second control circuit 125, an antenna 135electronically coupled to the transceiver 130 and/or 135 and disposed onthe substrate 110, an optional energy harvesting device 140 disposed onthe substrate, a first excitation device 150B, a first sensor 160B, asecond excitation device 155B, and a second sensor 165B. In some cases,the energy harvesting device 140 is electronically coupled to theantenna 135. In some cases, the wireless sensing device 100C includes asecond antenna connected to the second transceiver 132 while the antenna135 is connected to the first transceiver 130. Components with samelabels can have same or similar configurations, compositions,functionality and/or relationships as the corresponding components inFIGS. 1A and 1B.

One or more components in any one of wireless sensing devices describedherein, for example, control circuit, transceiver, thermal source,excitation device, sensor, energy harvesting device, and energy storagedevice can be made into an integrated circuit encapsulated within aelectronic package. In some implementations, the wireless sensingdevices described herein are passive sensing devices that do not includeactive power components (e.g., battery). In some other implementations,the wireless sensing devices described herein are active sensing devicesthat include active power components. In some cases, the embodiments ofwireless sensing device described herein are built into a singleelectronic package. In some cases, these wireless sensing devices can bebuilt into a NFC or RFID (radio frequency identification) tag as anaddressable sensor.

The embodiments illustrated in FIGS. 1B and 1C having two or moresensors, may have a number of benefits, such as tag size reduction,simplification of manufacturing, multiple sensing circuits having accessto the same power/magnetic field levels or a predefined ratiopower/magnetic field levels, construction of one device having multiplesensors enabling differential sensing architecture and/or enablingspatial mapping/sensing. In the embodiment illustrated in FIG. 1C, theutilization of a single antenna 135 for multiple sensing circuits caneliminate magnetic coupling detuning of two closely spaced antennaelements. While FIGS. 1B and 1C illustrated two sensors and/orexcitation devices in the wireless sensing device, a person skilled inthe art should readily design a wireless sensing device having more thantwo sensors and/or excitation devices.

FIG. 2A is a simplified schematic of one embodiment of an RF sensor tag200; FIG. 2B is a cross sectional view of the wireless sensing device200 at arrow 2B. The RF sensor tag 200 includes a substrate 210, anantenna 220 disposed on the substrate 210, an optional thermal spreader230, and a sensing circuit 240 electronically coupled to the antenna220. The sensing circuit 240 is disposed in the thermal spreader 230. Insome embodiments, the sensing circuit 240 can include a transceiver, amemory storing a unique identifier, a sensing element, and a heatingelement for heating a target area. In some other embodiments, thesensing circuit 240 includes an energy harvesting device. In some cases,only part of the sensing circuit 240 is disposed in the thermal spreader230. In some cases, the sensing element is thermally coupled to theheating element for sensing a temperature of the heating element, suchthat when the heating element is thermally coupled to a target area, theRF sensor tag 200 wirelessly receives a first power having a first formfrom a transceiver, the sensing circuit 240 transforms the first powerto a second power having a second form different from the first form anddelivers the second power to the heating element, the sensing elementsenses a time variation of the heating element temperature, and the RFsensor tag 200 wirelessly transmits to the transceiver a thermalcharacteristic of the target area based on the sensed time variation ofthe heating element temperature. In some cases, the first form could bea circulating alternating current and alternating voltage induced by analternating magnetic field. In some cases, the second form could be arectified version of the alternating voltage and current. In someembodiments, filtering of a rectified voltage and current by a capacitoror other means can produce approximately direct current and voltage asthe second form. The second form can alternatively be an alternatingcurrent and voltage transformed by the sensing circuit to a differentmagnitude, frequency, and/or phase from the first form.

In some cases, the substrate 210 is flexible and/or stretchable. In somecases, the RF sensor tag 200 includes an integrated circuit (IC)comprising at least part of the sensing circuit 240. In such cases, theantenna has a length between first and second ends and the IC iselectrically connected to the first and second ends of the antenna. Insome cases, the IC includes the memory, the wireless transceiver and theheating element. In some other cases, the IC includes the memory, thewireless transceiver and the sensing element. In yet other cases, the ICincludes the memory, the wireless transceiver, the heating element, andthe sensing element.

In some embodiments, the thermal spreader 230 is disposed on a majorsurface of the IC and adapted to substantially uniformly distribute heatfrom the heating element across the target area, where the major surfaceof the IC is a major top surface 242 and a major bottom surface of theIC 241. In some cases, the thermal spreader has a top surface 232 incontact with the bottom surface of the IC and an opposing bottom surface231 for thermally contacting the target area, the bottom surface 241 ofthe IC and the top surface 232 of the heat spreader 230 substantiallyoverlapping one another. In some cases, an area of the bottom surface231 of the thermal spreader 230 is greater than an area of the topsurface 232 of the thermal spreader 230. In some other cases, an area ofthe bottom surface 231 of the thermal spreader 230 is smaller than anarea of the top surface 232 of the thermal spreader 230.

In some cases, the heating element is also the temperature sensingelement. In some implementations, the first form of power is an AC formand the second form is a DC form. In some cases, the second formcomprises a rectified representation of the first form. In some cases,the sensing circuit 240 controls a magnitude of the second power.

In some embodiments, the RF sensor tag wirelessly receives an unknownfirst power having a first form from a wireless transceiver, and whereinthe electronic circuit transforms the unknown first power to a knownsecond power having a second form different from the first form. In somecases, the sensing element senses a time variation of the heatingelement temperature by generating a signal that has a known relationshipto the heating element temperature. In some cases, the sensing elementsenses a time variation of the heating element temperature by generatinga signal that is substantially proportional to the heating elementtemperature. In some implementations, when the sensing circuittransforms the first power to the second power, the sensing circuit isadapted to reduce a magnitude of the second power if the second power isgreater than a maximum threshold value. In some cases, the sensingcircuit is adapted to change the magnitude of the second power bychanging a resonant frequency of the RF sensor tag. In some embodiments,the thermal characteristic of the target area wirelessly transmitted tothe transceiver includes a thermal conductivity of the target area, athermal diffusivity of the target area, and/or a heat capacity of thetarget area.

In some embodiments, the RF sensor tag 200 is adapted to wirelesslycommunicate with a remote transceiver emitting power at a first radiofrequency, where the sensing circuit 240 is adapted to detune a resonantfrequency of the RF sensor tag 200 away from the first radio frequencyto control a magnitude of the first power received by the RF sensor tagfrom the remote transceiver. In some cases, the RF sensor tag 200 isadapted to wirelessly communicate with a remote transceiver emittingpower at a first radio frequency, where the sensing circuit 240 isadapted to tune a resonant frequency of the RF sensor tag away from thefirst radio frequency and tune the detuned resonant frequency back tothe first radio frequency. In some cases, the RF sensor tag 200 isadapted to wirelessly communicate with a remote transceiver emittingpower at a first radio frequency, such that if a resonant frequency ofthe RF sensor tag 200 drifts away from the first radio frequency, thesensing circuit 240 is adapted to tune the drifted resonant frequency ofthe RF sensor tag 200 back to the first radio frequency.

Generally, maximum power transfer from the remote transceiver to theRFID tag occurs when the resonant frequency of the RFID tag is the sameas the frequency at which power is emitted from the remote transceiver.In some cases, when the RFID tag is in close proximity to the remotetransceiver, more power may be transferred to the RFID tag than isneeded by the RFID tag. In this case, the RFID tag may sense theavailability of excess power and react by detuning the RFID tag resonatefrequency from the frequency at which power is emitted from the remotetransceiver, thus reducing the power available to the RFID tag byreducing the efficiency by which power is transferred from the remotetransceiver to the RFID tag. Detuning causes the resonate frequency ofthe RFID tag to be different than the frequency at which power isemitted from the remote transceiver, with the detuned resonate frequencyof the tag being at a frequency that is greater or less than thefrequency of the frequency at which power is emitted from the remotetransceiver. In this example, the resonate frequency of the RFID tag isdependent on a tuning capacitance of the RFID tag that resonates withthe inductance of a loop antenna of the RFID tag. As such, the resonatefrequency of the RFID tag can be modified by modifying the value of thiscapacitance. This capacitance can be modified by electronically couplingadditional capacitance in parallel with a base value of thiscapacitance, or electronically disconnecting parallel capacitance fromthis base value of capacitance. In an alternative configuration, thisbase value of capacitance could be modified by coupling a varactor diodein parallel with this base value of capacitance and modifying thecapacitance of the varactor diode by modifying a DC bias present acrossthe varactor diode.

Another means of detuning the RFID tag to reduce the efficiency by whichpower is transferred from the remote transceiver to the RFID tag is toreduce the Q factor of the RFID tag, for example by reducing the Qfactor of the RFID tag antenna. The Q (or quality factor) of the RFIDtag antenna is the ratio of energy stored in the antenna to the energydissipated by antenna, where the energy can be stored as a magneticfield and dissipated as heat due to the electrical resistance of theantenna. While many parameters contribute to the efficiency of powertransfer from a remote transceiver to a RFID tag, the Q factor of theRFID tag antenna can in some cases have a direct influence on theefficiency of power transfer. The Q factor of the RFID tag antenna canbe reduced by coupling an additional electrical resistance in serieswith the RFID tag antenna or by coupling an electrical resistance inparallel with the RFID tag antenna. This resistance can be controlled bya controller circuit. In some cases, when the RFID tag is in closeproximity to the remote transceiver, more power may be transferred tothe RFID tag than is needed by the RFID tag. In this case, the RFID tagmay sense the availability of excess power and react by reducing theefficiency of power transfer from the remote transceiver to the RFID tagby reducing the Q factor of the RFID tag by reducing the Q factor of theRFID tag antenna by modifying a resistance that is coupled to the RFIDtag antenna. An electronically controlled resistance that is coupled tothe RFID tag antenna could be implemented with a field effecttransistor, varactor diode, transistor switch, or any analog or digitalmeans of controlling a resistance.

FIG. 2C is a simplified schematic of one embodiment of a wirelesssensing device 200C. The wireless sensing device 200C includes asubstrate 210, an antenna 220 disposed on the substrate 210, a controlcircuit 240C, a thermal spreader 230C, and a sensing circuit 250Celectronically coupled to the control circuit 240C. The sensing circuit250C is disposed in the thermal spreader 230C. In some embodiments, thesensing circuit 250C can include a sensor and a thermal source. In somecases, the control circuit 240C regulates the thermal source in thesensing circuit 250C.

In one example illustrated in FIG. 2D, the control circuit 240Ccomprises a power measurement circuit to facilitate regulating thethermal source. The power measure circuit includes a controller 242C, apower source 244C, a voltage sensor 248C connected to the thermalsource, a current sensor 246C connected to the thermal source. The powerdelivered to the thermal source is calculated by multiplying the sensedcurrent by the sensed voltage. If this calculated power is above orbelow the desired power level, the power delivered by the power sourceto the thermal source is accordingly modified by the controller.

FIGS. 3A-3C illustrate simplified schematics of some embodiments ofwireless sensing device with multiple sensors; and FIG. 3D is across-sectional view of the wireless sensing device illustrated in FIG.3A. The RF sensor tag 300A, or referred to as wireless sensing device,as illustrated in FIG. 3A, includes a substrate 310, an antenna 320, anoptional first thermal spreader 330, a first sensing circuit 340 andelectronically coupled to the antenna 320, an optional second thermalspreader 350, and a second sensing circuit 360 electronically coupled tothe antenna 320. The first thermal region 330 and the second thermalregion 350 are thermally isolated from each other. The first and/orsecond sensing circuit (340, 360) can include one or more components oftransceiver, control circuit, energy harvesting device, energy storagedevice, thermal source, and sensor. In one embodiment, the sensingcircuit 340 provides a reference sensing signal, while the sensingcircuit 360 is in thermal contact with the object of interest andprovides sensing signals indicating temperature. In some cases, theinduced heating will lead to a larger temperature rise on the thermallyisolated region 330 than on the thermal region 350 that is in thermalcontact with the object, allowing differential measurement that accountsfor the variation in input power to the thermal source. For example, inthe case of an a wireless sensing device comprising an RFID tag for usewith an RF reader, the input power available may vary with RF readermagnetic field parameters, RFID tag resonance frequency relative to theRF reader frequency, variation in parameters with environmental factors,or other factors.

In some cases, the first sensing circuit 340 includes a first IC 342disposed on the substrate and the second sensing circuit 360 includes asecond IC 362, where each IC is electrically coupled to the antenna 320.In some implementations, the first sensing circuit 340 includes a firstheating element 344 and the second sensing circuit 360 includes a secondheating element 364, where each heating element heats a respective firstand second target area and is electrically coupled to the respectivefirst and second ICs (342, 362). In some cases, each of the first andsecond target areas has a thermal characteristic, where the thermalcharacteristic of the first target area is known and the thermalcharacteristic of the second target area is unknown. In some cases, thefirst target area is disposed on the substrate 310 and thermally coupledto the first heating element 344, where the first heating element andthe first target area are thermally isolated from the second heatingelement and adapted to be thermally isolated from the second targetarea.

In some embodiments, the first sensing circuit 340 includes a firsttemperature sensing element 346 and the second sensing circuit 360includes a second temperature sensing element 366, where each sensingelement (346, 366) is thermally coupled to the respective first andsecond heating elements (344, 364) for sensing a temperature of thecorresponding heating element (344, 364).

In some embodiments, when the second heating element 364 is thermallycoupled to the second target area, the RF sensor tag 300A wirelesslyreceives an input power having an input form from a transceiver, thefirst and second ICs (342, 362) transform the input power to respectivefirst and second powers having respective first and second formsdifferent from the input form and deliver the first and second powers tothe corresponding heating element (344, 364). In some cases, the firstand second sensing elements (346, 366) sense a time variation of thecorresponding heating element temperature, and the RF sensor tag 300Awirelessly transmits to the transceiver a thermal characteristic of thesecond target area based on comparing the time variation of the firstand second heating elements temperatures. In some cases, the RF sensortag 300A includes an IC comprising the first and second ICs (342, 362).

In some cases, the first power and the second power has a known ratio toeach other. For example, the magnitude of the first power is equal tothe magnitude of the second power. As another example, the magnitude ofthe first power is one third of the magnitude of the second power. Insome cases, the first power and/or the second power has a known ratio tothe input power. For example, the magnitude of the first power is onethird of the magnitude of the input power. In some embodiments, theinput power is in AC form. In some cases, the first form and/or thesecond form is an AC form. In some other cases, the first form and/orthe second form is a DC form.

In the example illustrated in FIG. 3B, the wireless sensing device 300Bincludes a substrate 310, an antenna 320 disposed on the substrate 310,a first thermal spreader 330, a first sensing circuit 340 disposed inthe first thermal spreader 330 and electronically coupled to the antenna320, a second thermal spreader 350, and a second sensing circuit 360, athird sensing circuit 362, and a fourth sensing circuit 364 disposed inthe second thermal spreader 350. The first thermal spreader 330 and thesecond thermal spreader 350 are thermally isolated. The sensing circuits360, 362, and 364 are spatially separated. The sensing circuits (340,360, 362, and 364) can include one or more components of transceiver,control circuit, thermal source, energy harvesting device, energystorage device, and sensor. In one embodiment, the sensing circuit 340provides a reference sensing signal, while the sensing circuits 360,362, and 364 are in thermal contact with the object of interest andprovides sensing signals indicating temperatures of various parts of theobject. In some cases, the sensing circuits 360, 362, and 364 can beplaced at any desired location or arbitrary location on a surface or inthree-dimensional space.

In the example illustrated in FIG. 3C, the wireless sensing device 300Cincludes a substrate 310, an antenna 320 disposed on the substrate 310,a first sensing circuit 360C and a second sensing circuit 366C. In someembodiments, the wireless sensing device 300C includes a sensing region350. The first and/or second sensing circuit (360C, 366C) can includeone or more components of transceiver, control circuit, energyharvesting device, energy storage device, thermal source, and sensor. Insome embodiments, the first sensing circuit 360C and the second sensingcircuit 366C have a known relative placement. In some cases, the firstsensing circuit 360C includes a thermal source and a sensor, while thesecond sensing circuit 366C includes a sensor but not a thermal source.In such cases, the second sensing circuit 366C can provide a measurementindicating a response to the thermal source activation in the firstsensing circuit 360C, in either time domain or frequency domain.

FIG. 4A illustrates a simplified schematic of one embodiment of awireless sensing device with multiple sensors and/or multiple RFdevices. The RFID tag 400A, as illustrated in FIG. 4A, includes asubstrate 410, a first RF device 412, and a second RF device 414, whereboth RF devices 412 and 414 are disposed on the substrate 410. The firstRF device 412 includes a first antenna 420 and a first circuit 440electronically coupled to the first antenna 420. In some cases, thefirst circuit 440 is disposed in an optional first sensing region 430.Similarly, the second RF device 414 includes a second antenna 425 and asecond circuit 445 electronically coupled to the second antenna 425. Insome cases, the second circuit 445 is disposed in an optional secondsensing region 435. The first and/or second circuit (440, 445) caninclude one or more components of transceiver, control circuit, energyharvesting device, energy storage device, excitation device, and sensor.In one embodiment, the first and second circuits (440 and 445) providethe sensor data of differential or spatial phenomena, where the spatialdistribution of the sensors can be controlled via the configuration ofthe RFID tag 400. The embodiment illustrated in FIG. 4A shows twoantennas disposed on a same planar surface. In some cases, two or moreantennas coupled with sensing circuits can be disposed on differentsurfaces, or in a manner that one antenna overlaps with another antenna.

In the example illustrated in FIG. 4A, the wireless sensing deviceincorporates two resonant circuits, which can change the resonatingfrequency. For example, for two loop antennas with one or more turns ofa conductor, in a planar coil configuration, in proximity to oneanother, the magnetic coupling k, of the two distinct loop antennas cancause resonance to occur at a lower frequency, as illustrated in FIG.8A. When designing a wireless sensing device containing two resonantcircuits, the magnetic coupling can be controlled by the relativeorientation between the two circuits. Because the coupling can becontrolled, the electronic components can be chosen such that theresultant resonant frequency is within a desired frequency range. Forexample, for a single resonant circuit of load capacitance of 50 pF, anda desired resultant resonant frequency of 13.56 MHz, Table 1 shows theshift in resonating frequency and inductance change based upon magneticcoupling (k).

TABLE 1 Desired Single Circuit Inductance of Capacitance FrequencyMagnetic Frequency Each Antenna (pF) (MHz) Coupling, k (MHz) (uH) 5013.56 0 13.56 2.76 50 13.56 0.3 15.46 2.12 50 13.56 0.5 16.61 1.84 5013.56 0.7 17.68 1.62

In some embodiments, the first and second antennas (420, 425) aremagnetically coupled to one another. In some cases, the RFID tag 400A isintended to have a pre-determined resonant frequency, each one of thefirst and second RF devices (412, 414) in the absence of the other oneis designed to have a resonant frequency different from thepre-determined frequency resulting in the RFID tag 400A having thepre-determined resonant frequency. In some cases, a magnitude of amagnetic coupling factor of the magnetically coupled first and secondantennas (420, 425) is at least 0.1. In some cases, a magnitude of amagnetic coupling factor of the magnetically coupled first and secondantennas (420, 425) is between 0.1 and 0.9. In some cases, the resonantfrequency of each distinct RF device (412, 414) is different from thetag resonant frequency by at least 5%. In some cases, the RF devices(412, 414) have a same resonant frequency. In one embodiment, the firstand second RF devices (412, 414) are configured to wirelesslycommunicate different first and second information from respective firstand second circuits (440, 445) to a same remote transceiver. In oneconfiguration, at least one IC (440 or 445) in the plurality of ICselectrically coupled to only one antenna in the plurality of antennas.

In one embodiment, the first and second circuits (440, 445) areintegrated circuits (ICs). In some cases, the first and second antennas(420, 425) are electrically coupled to respective first and secondintegrated circuits (ICs) (440, 445) disposed on the substrate.

In some cases, the first and second antennas (420, 425) are electricallycoupled to a same integrated circuits (IC) disposed on the substrate. Insome cases, each of the ICs (440, 445) have a distinct identificationnumber.

In some configurations, the first and second antennas (420, 425) arevertically offset relative to one another in a direction perpendicularto the substrate. In some cases, each of the first and second antennas(420, 425) is substantially overlapping the other of the first andsecond antennas. In some embodiments, the first and second antennas(420, 425) are substantially identical.

In the example illustrated in FIG. 4B, in plane view, the first andsecond antennas (420, 425) overlap one another. In some configurations,the substrate 410 has a top surface area enclosed by an outermostperimeter of the substrate, and in plane view, the first and secondantennas (420, 425) extend over a majority of the top surface area ofthe substrate.

FIG. 5 illustrates a simplified schematic of one embodiment of awireless sensing device with multiple sensors and a single antenna. Thewireless sensing device 500 includes a substrate 510, an antenna 520, afirst control circuit 530 electronically coupled to the antenna 520, afirst sensing circuit 540 electronically coupled to the first controlcircuit 530, a second control circuit 550 coupled to the antenna 520,and a second sensing circuit 560 electronically coupled to the secondcontrol circuit 550. The first and/or second control circuit (530, 550)can include one or more components of transceiver, microprocessor, amemory storing a unique identifier, an energy harvesting device, anenergy storage device. The first and/or second sensing circuit (540,560) can include one or more components of excitation devices andsensors. In the embodiment of the sensing circuit including anexcitation device, the sensor in the sensing circuit generates sensingsignals before and/or after the excitation device is activated. In someembodiments, the first and second sensing circuits (540 and 560) providesensing signals in response to the sensor data of differential orspatial phenomena, where the spatial distribution of the sensors can becontrolled via the configuration the wireless sensing device 500.

FIG. 6A illustrates one embodiment of a mobile sensing system 600. Themobile sensing system 600 includes a mobile device 610 and one or morewireless sensing devices 620. The wireless sensing device 620 can useany one or combination of the wireless sensing device configurationsdescribed in the present disclosure. In the embodiment illustrated, thewireless sensing device 620 includes an antenna 635, an energyharvesting device 630, an excitation device 640, and a sensor 650. Insome cases, the energy harvesting device 630 is electronically coupledto the excitation device 640 to provide power to the excitation device640. In some embodiments, the wireless sensing device 620 is configuredto measure a thermal property of the object and transmit a data signalassociated with temperature when the wireless sensing device isinterrogated. For example, the wireless sensing device 620 is in thermalcontact with an object of interest. As another example, the wirelesssensing device 620 is a wearable electronic device that will be in closeproximity with human skin when it is worn. A reader 618 is connected toor integrated with the mobile device 610, which is configured tointerrogate the wireless sensing device and receive the data signal. Theprocessor (not illustrated in FIG. 6A) in the mobile device 610 iselectronically coupled to the reader. The processor is configured todetermine the thermal property of the object based on the data signal.

In some embodiments, the excitation device can be regulated by powermodulation from the energy harvesting device 630 or an intentionalradiation source. In some cases, the wireless sensing device 620 is aradio frequency (RF) sensing device and the reader 618 is an RF reader.In some implementations, the RF reader can alter the duty cycle and/oramplitude of its electromagnetic field output to selectively change theamount of power applied to the wireless sensing device 620. As anotherexample, the mobile device 610 may provide a light source to thewireless sensing device 620. In such example, a mobile device LED canalter the duty cycle or amplitude of light output directed to thewireless sensing device 620. Such modulation can be done based onsensing information or power information or both communicated back tothe reader 618 or mobile device 610. Alternatively, such modulation canbe done based on measurements of the impedance by an RF reader. In somecases, various parameters of measured impedance such as resonancefrequency, resonance quality factor, and maximum value of the impedancemagnitude can be used to infer the amount of power being transferredinto the wireless sensing device 620; this inference could be importantdue to variables such as coupling between reader and circuit based ongeometry, alignment, and relative orientation, and on changes of theresonance parameters due to environmental factors.

In the example of FIG. 6A, mobile device 610 is illustrated as a mobilephone. However, in other examples, mobile device 610 may be a tabletcomputer, a personal digital assistant (PDA), a laptop computer, a mediaplayer, an e-book reader, a wearable computing device (e.g., a watch,eyewear, a glove), or any other type of mobile or non-mobile computingdevice suitable for performing the techniques described herein.

FIG. 6B illustrates a block diagram illustrating an example of a mobilesensing system including a mobile device 610 and a wireless sensingdevice 620 that operates in accordance with the techniques describedherein. For purposes of example, the mobile device of FIG. 6B will bedescribed with respect to mobile device 610 of FIG. 6A, and componentsfor the wireless sensing device 620 with same labels can have same orsimilar configurations, compositions, functionality and/or relationshipsas the corresponding components in FIG. 6A.

In this example, mobile device 610 includes various hardware componentsthat provide core functionality for operation of the device. Forexample, mobile device 610 includes one or more programmable processors670 configured to operate according to executable instructions (i.e.,program code), typically stored in a computer-readable medium or datastorage 668 such as static, random-access memory (SRAM) device or Flashmemory device. I/O 676 may include one or more devices, such as akeyboard, camera button, power button, volume button, home button, backbutton, menu button, or presentation device. Mobile device 610 mayinclude additional discrete digital logic or analog circuitry not shownin FIG. 6B.

In general, operating system 664 executes on processor 670 and providesan operating environment for one or more user applications 677 (commonlyreferred to “apps”), including sensor application 678. User applications677 may, for example, comprise executable program code stored incomputer-readable storage device (e.g., data storage 668) for executionby processor 670. As other examples, user applications 677 may comprisefirmware or, in some examples, may be implemented in discrete logic.

In operation, mobile device 610 receives data from the wireless sensingdevice 620. For example, reader 618 may interrogate the wireless sensingdevice 620 and receive sensing signals and provide the sensing signalsto the processor 670. In general, mobile device 610 stores the sensordata in data storage 668 for access and processing by sensor application678 and/or other user applications 677.

Thermal properties of an object or a material can be determined based ondata collected by thermal sensors, for example, in a sensing systemillustrated in FIG. 6A, or a sensing device illustrated in FIG. 1A. Forexample, the thermal conductivity, thermal diffusivity, and heatcapacity of a material in thermal contact with a thermal source can bedetermined by knowing the input power and the temperature profile as afunction of time along with calibration information including ananalysis framework and parameters related to geometry, other materialproperties, and the like. The effective thermal properties of compositematerial can similarly be determined.

One method to calculate the thermal conductivity and thermal diffusivityof material in thermal contact with a thermal source is by using thetransient plane source (TPS) analysis. This method is commonly employedwhen the thermal source can be represented as a plane source. Theexperiment consists of applying power to a thermal source and measuringthe power and temperature-time profile. An example measuredtemperature-time profile is shown in FIG. 8B. The transient heating of asquare plane has been shown to follow Equation (1):

$\begin{matrix}{{{\Delta{T(\tau)}} = {\frac{P_{o}}{4a*sq{{rt}(\pi)}*k}{H(\tau)}}},} & (1)\end{matrix}$where ΔT(τ) is the average temperature rise of the heater, P_(o) is theapplied power, 2a is the length of one side of the square thermalsource, k is the isotropic thermal conductivity of material in thermalcontact of the heater, H(τ) is the dimentionless specific time constant,and τ is defined in Equation (2):

$\begin{matrix}{{\tau = \left( \frac{\alpha\; t}{a^{2}} \right)^{\frac{1}{2}}},} & (2)\end{matrix}$where a is thermal diffusivity of the material is thermal contact withthe heater and t is time. The H(τ) is the dimentionless specific timeconstant and can be calculated as Equation (3):H(τ)=ƒ₀ ^(τ) dυ{erf(υ⁻¹)−π⁻¹/2υ[1−exp(−υ⁻²)]}²  (3)

The thermal conductivity can be determined from Equation (4):

$\begin{matrix}{{k = {\frac{P_{o}}{4a*{{sqrt}(\pi)}*{\Delta T}_{SS}}{H\left( {\tau = \infty} \right)}}},} & (4)\end{matrix}$where ΔT_(ss) is the steady state temperature change. According to FIG.8B, the steady state temperature change is the 10.59° C. With a=0.0025m, a constant applied power of 0.01 W, and H(τ=∞) the thermalconductivity of the surrounding material is calculated to be 0.044 W/mK.

The thermal diffusivity can be calculated from the full temperature-timeresponse through an iterative method to fit Equation (1) to the data setin FIG. 8B with a least-squares fitting method (other methods can beused). With this method, the thermal diffusivity was determined to be20×10⁻⁶ m²/s.

The heat capacity (C_(p)) of the surrounding material can be calculatedif the surrounding material's density is known according to Equation (5)

$\begin{matrix}{{C_{p} = \frac{k}{\rho\alpha}},} & (5)\end{matrix}$

For example, if the density of the surrounding material is 1200 g/m³,then the heat capacity is 1.83 J/gK of the surrounding material. Moreinformation on thermal property measurements can be found, for example,in a journal article by Gustafsson S. E., Transient plane sourcetechniques for thermal conductivity and thermal diffusivity measurementsof solid materials, Rev. Sci. Instrum., Volume 62, pp. 797-804, 1991,which is incorporated by reference in its entirety.

FIG. 6C illustrates one embodiment of a wireless sensing system 600C.The wireless sensing system 600C includes a reader 618C and one or morewireless sensing devices 620C (i.e., one sensing device illustrated),which can measure a physical property of an object. In some cases, thewireless sensing system 600C includes a computing device 610C, where thereader 618C is connected to or integrated with the computing device610C. The computing device 610C can include one or more processors,microprocessors, computers, servers, and other peripheral devices. Thewireless sensing device can use any one or combination of the wirelesssensing device configurations described in the present disclosure. Inthe embodiment illustrated, the wireless sensing device 620C includes awireless device 630C including a wireless transceiver 632C and anantenna 635C electronically coupled to the wireless transceiver 632C, anexcitation device 640C, and a sensor 650C electronically coupled to thewireless transceiver 632C. In some cases, the antenna 635C iselectronically coupled to the excitation device 640C to provide power tothe excitation device 640C. In some other cases, the reader 618Ctransmits an activation signal 615C to the wireless sensing device 620Cto activate the excitation device 640C. In some embodiments, the sensor650C generates a sensing signal associated with the physical property ofthe object, and the wireless transceiver 632C is configured to transmita data signal 613C associated with the sensing signal via the antenna635C. The reader 618C is configured to receive the data signal 613C. Insome embodiments, the computing device 610C is configured to determinethe physical property of the object based on the data signal 613C. Insome implementations, the reader 618C is further configured to adjustthe activation signal 615C based on the data signal 613C.

As an example, the wireless sensing device 620C includes a thermalsource 640C and a thermal sensor 650C, where the thermal source 640C andthermal sensor 650C are in thermal contact with the object of interest.In some cases, the wireless sensing device 620C can use a temperaturedependent resistor as both the thermal source 640C and the thermalsensor 650C. When power is delivered to this resistor, it producesthermal energy, and a measure of resistance of this same resistor can beused to measure temperature. It can be beneficial to deliver arelatively large amount of power to this resistor when using it toproduce a thermal excitation. Relatively little power can be deliveredto the resistor when measuring the resistance of this resistor tominimize heating during temperature measurements.

Controlling the power delivered to this resistor can be useful todetermine thermal properties of the object to which the wireless sensingdevice is attached. FIG. 8C is a concept example that indicates theusefulness of controlling the power delivered to a thermal source. Thisexample includes three sequential time intervals: time interval 1 from 0to 0.1 s, time interval 2 from 0.1 to 0.5 s, and time interval 3 from0.5 to 1.0 s. During time intervals 1 and 3, the power delivered to theresistor is relatively small, but adequate to measure the resistance todetermine temperature. During time interval 2, the power delivered tothe resistor is relatively large, i.e., 10 mW. During time interval 2the resistance of the resistor can be seen to increase from 10 ohms tonearly 11 ohms. During time interval 3 the resistance of the resistorcan be seen to decrease from nearly 11 ohms down to slightly above 10ohms. Thermal properties such as conductance, capacity and diffusivitycan be determined from these types of curves, such as using the TPSanalysis described.

In some embodiments, the reader 618C and/or a control circuit in thewireless sensing device 620C controls the magnitude of the excitation(i.e. power supply to the excitation device) over time. In some cases,the reader 618C and/or a control circuit in the wireless sensing device620C controls the excitation in response to the measured sensor signalchanges over time. For example, if the change in the sensor signal isnot adequate, the reader 618C and/or a control circuit in the wirelesssensing device 620C may increase the magnitude or duration of theexcitation; and if the change in the sensor signal is large, the reader618C and/or a control circuit in the wireless sensing device 620C mayreduce the magnitude or duration of the excitation, for example, toensure that the response remains within the dynamic range of the sensingsystem. In some cases, the reader 618C and/or a control circuit in thewireless sensing device 620C controls the excitation by providing aconstant power with known value or a known power-time profile.

FIG. 6D illustrates one embodiment of a wireless sensing system 600Dhaving more than one wireless sensing devices. The wireless sensingsystem 600D includes a reader 618D and three or more sensing devices620D. In some cases, the wireless sensing system 600D includes acomputing device 610D, where the reader 618D is connected to orintegrated with the computing device 610D. The computing device 610D caninclude one or more processors, microprocessors, computers, servers, andother peripheral devices. The sensing device 620D can use any one orcombination of the wireless sensing device configurations described inthe present disclosure. In some embodiments, the reader 618D isconfigured to transmit an activation signal to at least some of thesensing devices 620D to activate the excitation devices (notillustrated) in the sensing devices 620D. In addition, the reader 618Dis configured to receive data signals from the sensing devices 620D. Insome cases, at least some of the sensing devices 620D are disposed indifferent locations from each other. In some configurations, the reader618D is configured to coordinate the transmission of the activationsignal to each of the sensing devices 620D in a pattern related to thelocation of the sensing device 620D, for example, activate excitationdevices individually, simultaneously, or other temporal or spatialpatterns.

In some embodiments, at least some of sensing devices 620D are disposedproximate to an object, and the computing device 610D is configured todetermine the physical property of the object based on the data signalsgenerated by the sensing devices 620D. In some cases, at least one ofthe sensing devices 620D is configured to transmit a reference datasignal corresponding to a reference sensor signal independent from thephysical property of the object, and the computing device 610Ddetermines the physical property of the object using the reference datasignal.

In some embodiments, some of the sensing devices 620D include onlyexcitation device but no sensor, also referred to as actuation device.In some cases, an actuation device is disposed at a different locationfrom a sensing device that includes a sensor. In such embodiments, thereader 618D is configured to transmit an activation signal 615D to theactuation device to activate the excitation device and receive the datasignal 613D from the sensing device.

FIG. 7A illustrates an example flowchart for the operation of oneembodiment of a wireless sensing device and/or system, for example, thewireless sensing device illustrated in FIG. 1A or the wireless sensingsystem illustrated in FIG. 6A. First, the energy harvesting devicereceives and converts power (step 710A). Next, the sensor generates aninitial sensor signal indicating temperature T(0) (step 715A). Theenergy harvesting device provides power to the thermal source (step720A), which may also occur nearly simultaneously with step 715A ineither order. Then, the sensor generates sensor signal indicatingtemperature T(n) (step 725A). The control circuit in the wirelesssensing device or a computing device that receives the sensor signalcomputes a signal indicating temperature difference ΔT=T(n)−T(0) and asignal indicating temperature rate of change dT/dn=T(n)−T(n−1) (step730A). The control circuit or the computing device determines whether athermal steady state is reached, where dT/dn is less than or equal to apredetermined threshold and/or other conditions. If the steady state isnot reached, the sensor continues to generate sensor signal T(n)indicating temperature (step 725A). If the steady state is reached, thecontrol circuit or the computing device computes thermal property basedon ΔT (step 750A), and may deactivate the thermal source.

FIG. 7B illustrates an example flowchart for the operation of oneembodiment of a wireless sensing device or system having two sensors,for example, the wireless sensing device illustrated in FIG. 1B or thewireless sensing system illustrated in FIG. 6D. In some embodiments,more than two sensors can be included in the wireless sensing device orsystem to determine thermal property using similar steps. First, theenergy harvesting device receives and converts power (step 710B). Next,two sensors generate an initial sensor signals indicating temperatureT₁(0) and T₂(0) (step 715B). The energy harvesting device provides powerto the thermal source(s) (step 720B), which may also occur nearlysimultaneously with step 715B in either order. Then, the sensorsgenerate sensor signals indicating temperatures T₁(n) and T₂(n)indicating temperature (step 725B). The control circuit in the wirelesssensing device or a computing device that receives the sensor signalscomputes signals indicating temperature differences relative to initialtemperatures measured by the two sensors respectively (ΔT₁=T₁(n)−T₁(0),ΔT₂=T₂(n)−T₂ (0)) and signals indicating temperature change ratesmeasured by the two sensors respectively (dT₁/dn=T₁(n)−T₁(n−1),dT₂/dn=T₂ (n)−T₂ (n−1)) (step 730B). The control circuit or thecomputing device determines where a thermal steady state is reached,where dT₁/dn and/or dT₂/dn are less than a predetermined thresholdand/or other conditions. If the steady state is not reached, the sensorscontinue to generate sensor signals T_(1,2) (n) (step 725B). If thesteady state is reached, the control circuit or the computing devicecomputes one or more thermal properties based on ΔT_(1,2) (step 750B),and may deactivate the thermal source.

FIG. 9A illustrates one embodiment of hydration sensing system 900. Thehydration sensing system 900 includes a computing device 910, a reader918 and one or more wireless sensing devices 920, which can be disposedin thermal contact with the skin of a person 960 or can be used todetermine liquid content of a material. In some cases, the reader 918 isconnected to or integrated with the computing device 910. The computingdevice 910 can include one or more processors, microprocessors,computers, servers, and other peripheral devices. The wireless sensingdevice 920 can use any one or combination of the wireless sensing deviceconfigurations described in the present disclosure. In the embodimentillustrated, the wireless sensing device 920 includes a substrate 930, aRF circuit 932, an antenna 935 disposed on the substrate 930 andelectronically coupled to the RF circuit 932, a thermal source 940, anda sensor 950 thermally coupled to the thermal source 940 for sensing atemperature of the thermal source 940. In some embodiments, when thethermal source 940 is thermally coupled to a target area, the wirelesssensing device 920 wirelessly receives a first power having a first formfrom a transceiver, the RF circuit 932 transforms the first power to asecond power having a second form different from the first form anddelivers the second power to the thermal source 940, the sensor 950senses a time variation of the thermal source temperature, and the RFcircuit 932 wirelessly transmits the sensed time variation of thethermal source temperature. The reader 918 is configured to receive thesensed time variation of the thermal source temperature and thecomputing device 910 is configured to determine a hydration indicatorindicative of hydration level based on the sensed time variation of thethermal source temperature. In some embodiments, the wireless sensingdevice 920 includes a processor to determine a hydration indicatorindicative of hydration level based on the sensed time variation of thethermal source temperature.

FIG. 9B shows a schematic temperature-time profile of a thermal sourcebefore, during, and after a constant input power. The time scale andtemperature rise of the profile are a function of the applied power,shape and thermal properties of the thermal source, and thermalproperties of the surrounding material in thermal contact with thethermal source. For quantitative thermal measurements, the thermal andgeometrical properties of the thermal source are needed. Thetemperature-time profile can be separated into three distinct regions,as illustrated in FIG. 9B. The first region is the non-steady stateheating region where a temperature rise is observed. From this region,the thermal diffusivity of the surrounding material can be determinedfrom the rate of temperature increase. The second region of the profileis the steady state region; the region where a maximum and steady-statetemperature is reached. For this region, the thermal conductivity of thesurrounding material can be determined. The temperature at steady statevaries inversely with the thermal conductivity of the material. Thethird region is the non-steady state cooling region after removal of theapplied power to the thermal source. Similar to first region, the rateof temperature decrease can be used to measure the thermal diffusivityof the surrounding material. FIG. 9B shows a temperature-time profilefor a heater, but the embodiments described herein apply to a thermalsource as a cooler.

In some cases, the effective thermal properties of a material or anobject may be dependent on moisture content. For example, a region ofskin like that of a human or other mammal having biological cells andtissues along with a particular fraction of water, where the effectivethermal conductivity of the region of skin will vary relative to theratio of water to tissue in that region. The determination of themoisture (or hydration) level of the skin can be determined through alook-up table or an analytical equation based on the measured value(s)of thermal diffusivity, thermal conductivity, heat capacity, or acombinations thereof, or indices proportional to the value(s). Forexample, thermal conductivity of the dry human skin can be on the orderof 0.2-0.3 W/m-K; thermal conductivity of the normal human skin can beon the order of 0.3-0.4 W/mK; thermal conductivity of the hydrated humanskin can be on the order of 0.4-0.55 W/m-K; and thermal conductivity ofthe human perspiration can be in the range of 0.55-0.7 W/m-K.

For example, an analytical approach, discussed in Skin ThermalConductivity A Reliable Index of Skin Blood Flow and Skin Hydration, A.DITTMAR, Laboratory of Thermoregulation, U.A. 181 C.N.R.S., Lyon,France, Apr. 5, 1989 been reported to measure the moisture content ofskin from the thermal conductivity of the skin (k_(skin)) through theEquation (6)% water content=⅙(k _(skin)−%(lipids+protiens)1.8)  (6)

An example flowchart to determine a hydration level using any of thewireless sensing device or any of the wireless sensing system describedherein is illustrated in FIG. 7C. Some steps, for example, step 710C,step 727C, are optional steps of the sensing system. First, the wirelesssensing device generates a first sensor signal (step 710C), before thethermal source is activated or when thermal source is just activated.Next, the thermal source is activated (step 715C). The wireless sensingdevice generates a series of sensing signals (step 720C). The wirelesssensing device or system determines a thermal property based on at leastsome of the series of sensing signals (step 730C). The thermal propertycan include, for example, thermal conductivity, heat capacity, thermaldiffusivity, and the like. The wireless sensing device or system furthergenerates a hydration indicator indicative of hydration level of theobject based on the determined thermal property and a reference data(740C). The reference data can be an analytical function, a look-uptable, a matrix, a constant, or a combination thereof.

In some cases, the wireless sensing device or system evaluates whether athermal steady state is reached (step 723C), using the series of seriessignals, for example, the change rate of the sensor signals is smallerthan a predetermined threshold. In some cases, the wireless sensingdevice or system determines a thermal conductivity of the object basedon the first sensor signal and the sensor signal generated when or afterthe thermal steady state is reached, where the hydration indicator isgenerated based on the thermal conductivity. In some implementations,the thermal source is deactivated after the thermal steady state isreached (step 725C). The sensing device generates a series of coolingsensing signals after the thermal source is deactivated (step 727C). Insome cases, the wireless sensing device or system determines a thermaldiffusivity of the object based on at least some of the series ofcooling sensing signals, where the hydration indicator is generatedbased on the thermal diffusivity.

In some embodiments, as illustrated in FIGS. 9C and 9D, a wirelesssensing device 920C can be designed to measure a liquid level. Thewireless sensing device 920C includes an absorption element 945 that canabsorb liquid, such as sweat, wound exudate, condensate, perspiration,oil, or the like. In the embodiment as illustrated in FIG. 9C, theabsorption element 945 is in thermal contact with the thermal source 940that is thermally coupled to the sensor 950. FIG. 9D illustrates anotherexample of the wireless sensing device 920C includes the absorptionelement 945 that is in thermal contact with the thermal source 940 andthermal sensor 950, which can be an integrated component, for example.In some cases, the absorption element 945 and the thermal source940/thermal sensor 950 are disposed proximate to each other and formthermal contact. In some other cases, the absorption element 945 and thethermal source 940/thermal sensor 950 are in physical contact. In somecases, the thermal source 940/thermal sensor 950 is disposed on or atleast partially in the absorption element 945. Absorption element mayinclude absorption material(s), for example, such as porous material, anatural or synthetic sponge, water-absorbing gel, superabsorbentpolymer, a form, a gauze, a non-woven patch, or the like. Sponges may bemade from cellulose, polyester or other polymers. Superabsorbentpolymers may include polyacrylate/polyacrylamide copolymers, polyvinylalcohol copolymers, for example. The wireless sensing device 920C caninclude other components, for example, the components in the wirelesssensing device 920 illustrated in FIG. 9A.

In one embodiment, the wireless sensing device 920C is an RF sensor,which includes a substrate, an antenna disposed on the substrate, an RFcircuit electrically coupled to the antenna, the RF circuit comprising aprocessor; an absorption element (e.g., 945) comprising absorptionmaterial, a thermal source (e.g., 940) electrically coupled to the RFcircuit and thermally coupled to the absorption element; and a sensingelement (e.g., 950) thermally coupled to the thermal source for sensinga temperature of the thermal source, such that after the absorptionelement is used to absorb liquid, the RF sensor wirelessly receives afirst power having a first form from a transceiver, the RF circuittransforms the first power to a second power having a second formdifferent from the first form and delivers the second power to thethermal source, the sensing element senses a time variation of thethermal source temperature, and the processor determines an indicatorindicating liquid level based on the sensed time variation of thethermal source temperature. A wireless sensing device or system can usea similar flowchart as illustrated in FIG. 7C to gather sensor data anddetermine an indicator indicating a liquid level.

EXAMPLES Example 1 Wireless Sensing Device—Assembly and Temperature RiseRates

As illustrated in FIG. 10, a wireless sensing device 1000 was assembledin the following manner. A 5.0 mm×5.0 mm×0.9 mm AMS SL13A packaged RFIDintegrated circuit with temperature sensor 1020 obtained from Digikey ofThief River Falls, Minn. was electrically connected to a loop antenna1030 composed of solid enamel coated 26 AWG copper wire obtained fromDigikey of Thief River Falls, Minn. The loop antenna 1030 was createdwith four circular turns of the solid enamel coated 26 AWG copper wireat a diameter of 90 mm. The AMS SL13A packaged RFID integrated circuitwith temperature sensor 1020 was also electrically connected to a 1.0mm×0.5 mm×0.4 mm 0402-size resistor 1050 of value 850Ω obtained fromDigikey of Thief River Falls, Minn. The loop antenna 1030 was connectedto pins 5 and 6 (“ANT1” and “ANT2”) and the resistor was connectedacross terminals 3 (“VEXT”) and 12 (“VSS”) of the AMS SL13A packagedRFID integrated circuit with temperature sensor 1020.

The value of the surface mount resistor was chosen to limit the thermalsource current (I) to a maximum specified value of 4 mA when the voltage(V) reaches a maximum of 3.4 V. This occurred when the wireless sensingdevice 1000 was located in the maximum magnetic field emitted by areader and provided 13.6 mW (e.g., P=VI) of thermal source power in theresistor. Typically, a 0402-size resistor component dissipating 13.6 mWin a vacuum or with minimal heat transfer to the surrounding environmentwould experience an initial temperature rise rate of approximately 22.6°C. per second. Alternatively, if it were in similar environmentalconditions and thermally connected to the AMS SL13A packaged RFIDintegrated circuit with temperature sensor 1020, the initial temperaturerise rate would be reduced to about 0.4° C. per second due to the muchlarger volume of the SL13A package. The temperature rise rates werecalculated based on the heat capacity of aluminum oxide and silicon,respectively, by using Equation (6):

$\begin{matrix}{\frac{\Delta T}{\Delta t} = \frac{P}{\left( {c_{p} \times \rho \times V} \right)}} & (6)\end{matrix}$where ΔT is temperature in ° C., Δt is time in seconds, P is power injoules per second, c_(p) is specific heat capacity in joules per gram-°C., ρ is mass density in grams per cubic millimeter, and V is the volumein cubic millimeters. Calculation parameters and results are containedin Table 2.

TABLE 2 Parameter 0402 Resistor SL13A Package Power (J/s) 1.36E−21.36E−2 Specific Heat Capacity (J/g-° C.)  8.8E−1  7.0E−1 Density(g/cm³) 3.90 2.33 Thickness (mm)  3.5E−1  9.0E−1 Area (mm²)  5.0E−12.5E1 Volume (mm³) 1.75E−1 2.25E1  Volume (cm³) 1.75E−4 2.25E−2 ΔT/Δt (°C./s) 2.26E1  3.71E−1

An intended use of the wireless sensing device 1000 would be to measurethe effective local thermal conductance when in contact with a materialof interest (e.g., human skin). The induced steady-state temperaturerise in such a case would be the product of the input power P in wattsand thermal resistance R_(th) in ° C. per watt as represented inEquation (7):ΔT=P×R _(th)  (7)

With 13.6 mW of heating, and given a minimum relative temperaturemeasurement resolution (Minimum ΔT Resolution) of 0.3° C., the devicecould measure a thermal resistance (Minimum Measureable R_(th)) as lowas 22° C. per watt and as high a thermal resistance as the wirelesssensing device 1000 maximum temperature limit allow; for the AMS SL13Awhich has a temperature measurement limit of 60° C. in its standardmode, that maximum measureable thermal resistance for 13.6 mW of heatingis about 3000° C. per watt corresponding to a 40° C. steady-stateexcursion above an ambient temperature of 20° C. A smaller temperatureresolution or larger input power would reduce the minimum measureablethermal resistance as indicated in Table 3 and Table 4. The transientplane source method described previously herein is an example where thevalue of thermal resistance R_(th) is the inverse value of effectivethermal conductivity of the material scaled by lateral dimension of theheater element and constant factors.

TABLE 3 Power Input Minimum ΔT Resolution Minimum Measurable R_(th) (W)(° C.) (° C./W) 1.36E−2 3.0E−1 2.2E1  1.36E−2 1.0E−1 7.4 1.36E−2 5.0E−23.7 1.36E−2 2.0E−2 1.5 1.36E−2 1.0E−2 7.4E−1 1.36E−2 5.0E−3 3.7E−11.36E−2 2.0E−3 1.5E−1 1.36E−2 1.0E−3 7.0E−2

TABLE 4 Power Input Minimum ΔT Resolution Minimum Measurable R_(th) (W)(° C.) (° C./W) 1.0 1.0E−1 1.0E−1 2.0 1.0E−1 5.0E−2 5.0 1.0E−1 2.0E−21.0E1 1.0E−1 1.0E−2 2.0E1 1.0E−1 5.0E−3 5.0E1 1.0E−1 2.0E−3 1.0E2 1.0E−11.0E−3

Example 2 Wireless Sensing Device—Differential

A wireless sensing device with a first and second integrated circuit,each having a unique identification number was electrically connected toa single antenna as represented in FIG. 11. A wireless sensing device1100 was assembled in the following manner. Two 5.0 mm×5.0 mm×0.9 mm AMSSL13A packaged RFID integrated circuits with temperature sensors 1120,1122 obtained from Digikey of Thief River Falls, Minn. were electricallyconnected to a loop antenna 1130 composed of solid enamel coated 26 AWGcopper wire obtained from Digikey of Thief River Falls, Minn. through anFR4 interface board 1160 with copper pads and traces configured to allowparallel connection of the antenna 1130 and integrated circuits 1120,1122. Both of the AMS SL13A packaged RFID integrated circuits withtemperature sensors 1120, 1122 were connected to the interface board1160 with short lengths of solid enamel coated 34 AWG copper wireobtained from Digikey of Thief River Falls, Minn. The loop antenna 1130was created with four circular turns of the solid enamel coated 26 AWGcopper wire at a diameter of 60 mm. The loop antenna 1130 was designedto resonate with 50 pF with an intended inductance of 2.75 μH at 13.56MHz. Actual loop antenna inductance of the 60 mm diameter circular coilwas higher than intended and produced a resonance of about 13.17 MHz, soits inductance was reduced to achieve tag resonance near 13.56 MHz bycompressing the circular antenna into an ellipse as illustrated in FIG.11. The loop antenna 1130 was connected to pins 5 and 6 (“ANT1” and“ANT2”) of the AMS SL13A packaged RFID integrated circuits withtemperature sensors 1120, 1122 through the interface board 1160. Thethermal sources in this example are Joule heating within each integratedcircuit 1120, 1122 induced by interaction of each integrated circuitwith a reader magnetic field via the antenna.

The wireless sensing device 1100 was tested by a 3M radio frequencyidentification (RFID) reader Model 810 and reader antenna Model 870obtained from 3M Company of St. Paul, Minn. connected via a universalserial bus (USB) cable to a laptop personal computer (PC) running testsoftware. The test consisted of operating the reader via the PC to queryfor the presence of unique security identifiers (SIDs) using the ISO15693 RFID communication protocol. The software reported the number andquantity of unique SIDs read. The maximum read range was found to be 18cm for this configuration.

The wireless sensing device 1100 was also tested with a RFID reader AMSAS3911 General Purpose Demo Kit Rev 1.0 and its accompanying PC softwareobtained as part number AS3911-DK-ST-ND from Digikey of Thief RiverFalls, Minn., to produce the example temperature versus time data inFIG. 12. The thermal source within each integrated circuit wascontrolled by modulation of the reader magnetic field by placing thesensing device 1100 in proximity to the reader with the reader magneticfield disabled and the integrated circuits 1120, 1122 primarilysuspended in air, and then enabling the reader magnetic field at zerotime in FIG. 12 and querying a first temperature from each integratedcircuit 1120, 1122. The reader magnetic field remained enabled whiledata were periodically obtained by addressing one of the integratedcircuits with the reader, with the first temperature subtracted toproduce the relative temperature shown in FIG. 12. Data gathered asreported by alternately addressing the other integrated circuit weresimilar.

Example 3 Wireless Sensing Device—Hydration Monitoring (Single TurnLoop)

As illustrated in FIG. 10, a wireless sensing device 1000 was assembledin the following manner. A 5.0 mm×5.0 mm×0.9 mm AMS SL13A-AQFT packagedRFID integrated circuit with temperature sensor 1020 obtained fromDigikey of Thief River Falls, Minn. was electrically connected with 34AWG copper inductor wire to a 78 mm×84 mm×0.08 mm loop antenna 1030. Theloop antenna 1030 was created with a single loop of 3M™ copper tapeobtained from 3M™ Company of Saint Paul, Minn. Parallel capacitancetotaling 606 pF of 0603 size NPO type ceramic tuning capacitors obtainedfrom Digikey of Thief River Falls, Minn. was connected to the loopantenna. The wireless sensing device 1000 was affixed to a 100 mm×100mm×3.3 mm layer of foam from a 90612 Tegaderm™ foam adhesive dressingobtained from 3M™ Company of Saint Paul, Minn.

Dry and wet measurements were obtained using an LG Nexus 5 smartphonewith custom Android application that provided wireless power, analyzedtemperature data points, and calculated time-temperature thresholds aswell as indicated status of the Tegaderm™ Foam Adhesive dressing layer.Analysis was performed with the LG Nexus 5 smartphone in a lateralposition relative to the foam adhesive dressing at a fixed verticalseparation of 15 mm.

Measurements were performed dry and after dispensing controlled amountsof deionized water distributed across the bottom surface of the foam.Each measurement was repeated three times for each condition separatedby three minutes to allow the NFC integrated circuit to return to nearambient temperature. Results are contained in Table 5.

TABLE 5 Relative Temperature (ΔT) Dry 10 mL Water 20 mL Water Time (s)Avg. Max. Min. Avg. Max. Min. Avg. Max. Min.  0 0.0 0.0 0.0 0.0 0.0 0.00.0 0.0 0.0  2 0.6 0.7 0.3 0.6 0.7 0.3 0.3 0.3 0.3  4 1.6 1.7 1.3 1.01.3 0.7 0.6 0.7 0.3  6 2.0 2.4 1.7 1.3 1.3 1.3 0.8 1.0 0.7  8 2.7 2.72.7 1.8 2.1 1.7 0.9 1.0 0.7 10 3.2 3.4 3.1 2.0 2.4 1.7 1.1 1.3 1.0 205.2 5.4 5.1 3.1 3.4 2.7 1.3 1.3 1.3

The configuration was designed to be insensitive to changes in localpermittivity such as proximity to water. Measured antenna resonanceconditions did not change significantly during the test, indicating themechanism observed is based on changes in heat transfer rather thanchanges in input electrical power. Measured resonance for each conditionis in summarized in Table 6.

TABLE 6 Resonant frequency (MHz) Quality factor Dry 13.01 46 10 mL water13.00 46 20 mL water 12.92 45

Example 4 Wireless Sensing Device—Hydration Monitoring (Four Turn Loop)

As illustrated in FIG. 10, a wireless sensing device 1000 was assembledin the following manner. A 5.0 mm×5.0 mm×0.9 mm AMS SL13A-AQFT packagedRFID integrated circuit with temperature sensor 1020 obtained fromDigikey of Thief River Falls, Minn. was electrically connected to a loopantenna 1030 composed of solid enamel coated 34 AWG copper wire obtainedfrom Digikey of Thief River Falls, Minn. The loop antenna 1030 wascreated with four circular turns of the solid enamel coated 34 AWGcopper wire at a diameter of 50 mm. A 24 pF 0603 size NPO type ceramictuning capacitor obtained from Digikey of Thief River Falls, Minn. wasconnected to the loop antenna 1030. The wireless sensing device 1000 wasaffixed to a 100 mm×100 mm×3.3 mm layer of foam from a 90612 Tegaderm™Foam Adhesive dressing obtained from 3M™ Company of Saint Paul, Minn.

Dry and wet measurements were obtained using an LG Nexus 5 smartphonewith custom Android application that provided wireless power, analyzedtemperature data points, and calculated time-temperature thresholds aswell as indicated status of the Tegaderm™ foam adhesive dressing.Analysis was performed with the LG Nexus 5 smartphone in a lateralposition relative to the foam adhesive dressing at a fixed verticalseparation of 15 mm.

Measurements were performed dry and after dispensing controlled amountsof deionized water distributed across the bottom surface of the foam.Each measurement was repeated three times for each condition separatedby three minutes to allow the NFC integrated circuit to return to nearambient temperature. Results are contained in Table 7.

TABLE 7 Relative Temperature (ΔT) Dry 10 mL Water 20 mL Water Time (s)Avg. Max. Min. Avg. Max. Min. Avg. Max. Min.  0 0.0 0.0 0.0 0.0 0.0 0.00.0 0.0 0.0  2 0.4 0.7 0.3 1.0 1.0 1.0 0.2 0.3 0.0  4 1.3 1.7 1.0 1.11.3 1.0 0.4 0.7 0.3  6 2.2 2.4 2.1 1.7 1.7 1.7 0.4 0.7 0.3  8 2.7 3.12.4 2.0 2.4 1.7 0.4 1.0 0.0 10 3.2 3.4 3.1 2.4 2.7 2.1 0.4 0.7 0.3 205.6 5.7 5.4 3.4 3.7 3.1 0.8 1.0 0.7

Heat transfer effects are confounded by the shifts in resonance thatcause less electrical power to be transferred as water infiltrates thefoam. Measured resonance are summarized in Table 8.

TABLE 8 Resonant frequency (MHz) Quality factor Dry 14.42 73 10 mL 13.5727 20 mL 12.46 15

Example 5 Wireless Sensing Device—Sweat Monitoring

A wireless sensing device 620C, as represented in FIG. 6C, was assembledin the following manner. A STM20-DD9F ultra-low current precision analogtemperature sensor 650C obtained from STMicroelectronics of Geneva,Switzerland was thermally connected to a 500 resistive thermal source640C comprised of two 0402 sized 1000Ω resistors in parallel obtainedfrom Digikey of Thief River Falls, Minn. The temperature sensor 650C andresistive thermal source 640C were positioned on opposite surfaces of aninternally fabricated flexible circuit constructed with three 12 μmlayers: copper/polyimide/copper. A sensor tip for measurement wasaffixed with 9836 acrylate adhesive film obtained from 3M™ Company ofSaint Paul, Minn. to the bottom layer of foam at the central surface ofan assembly of two layers of 25 mm×25 mm MSX-6916B open-cellpolyurethane foam obtained from 3M™ Company of Saint Paul, Minn. whichhas nominal thickness 2.4 mm. The internal surface of the top layer ofthe two layers of foam was coated with 9836 acrylate adhesive filmobtained from 3M™ Company of Saint Paul, Minn. to protect from moistureinfiltration. A loop antenna 635C was created with two circular turns ofcopper traces with thickness 17 μm, trace width 3.4 mm, and outerdiameter of 50 mm on opposing surfaces of a printed circuit board. Aparallel capacitance of 350 pF comprised of 0603 size NPO type ceramictuning capacitors obtained from Digikey of Thief River Falls, Minn. wasconnected to the loop antenna 635C to produce a resonant frequency of13.66 MHz and quality factor of 50.

A dynamic NFC M24LR16E-RMC6T/2 transponder 632C with serialcommunication interface and non-volatile memory integrated circuit waselectrically connected to the loop antenna and to a STM32L052C8T6microcontroller, both obtained from STMicroelectronics of Geneva,Switzerland, were used for power and analysis purposes. The temperaturesensor and resistive thermal source were each electrically connected tothe microcontroller. An NFC CR95HF reader 618C demonstration boardobtained from STMicroelectronics, with corresponding M24LRxx ApplicationSoftware also obtained from STMicroelectronics, was positioned 25 mmabove the printed circuit board to gather measurements.

Table 9 represents the experimental conditions for deionized waterprogressively added to the bottom layer of foam. In each case, toemulate the uniform distribution of fluid, a controlled volume ofdeionized water was dispensed onto an aluminum plate. The bottom layerof foam was placed onto the water drop and then compressed and releasedmultiple times to distribute the fluid throughout the bottom foam layer.Then, the bottom layer of foam was weighed, reassembled with the toppiece of foam for thermal isolation from the ambient, and a wirelesslypowered measurement was performed with the NFC reader 618C and thewireless sensing device 620C. A measurement was triggered by the NFCreader 618C via an activation signal 615C in the form of wirelesslywriting to a memory location within the wireless transceiver 632C.Temperature and voltage applied to the thermal source resistance wererecorded at regular intervals in the wireless sensing device, anduploaded to the NFC reader as data signal 613C. Dataset ID “A6” wasmeasured after the foam was allowed to dry in ambient conditions for oneweek. The column of Table 9 for “Equivalent minutes at 0.7 mg/cm²/min”is the amount of time in minutes that each respective amount of waterwould take to accumulate at a uniform flow rate of 0.7 mg of water persquare centimeter per minute into a 25×25 mm surface of the bottom foam.The column of Table 9 for “Water % of Foam 1.5 cm³ Volume” is thepercentage of the bottom foam layer's volume assuming a water massdensity of 1 gram per cubic centimeter.

TABLE 9 Measured Water Bottom Mass Layer per Equivalent Foam WaterSurface minutes at Water % of Dataset Mass Mass Area 0.7 mg/ Foam 1.5cm³ ID (g) (g) (mg/cm²) cm²/min Volume A1 0.21 — — — — A2 0.31 0.10 1522  6% A3 0.39 0.18 29 41 12% A4 0.55 0.34 54 78 23% A5 0.82 0.61 98 13941% A6 0.22 0.01 2 2  1%

Results are shown in Table 10 and are presented as the relativetemperature per unit of average thermal power represents the reductionin the temperature rise with increasing fluid concentration due to theincrease in thermal conductivity and diffusivity. Relative temperatureΔT is defined as the change from the initial temperature value: T−T_(I),where T_(I) was the initial temperature. Recorded thermal power in thesedatasets was between 8.2 and 8.8 milliwatts and an average wascalculated for each dataset. The values in Table 10 were averaged fromthe raw data collected at 50 millisecond intervals; averaging windows ofone-second width were used, centered on each whole second.

TABLE 10 ΔT per average power (° C./mW) t (s) A1 A2 A3 A4 A5 A6  0.000.00 0.00 0.00 0.00 0.00 0.00  1.00 0.22 0.15 0.15 0.12 0.13 0.21  2.000.38 0.27 0.25 0.21 0.19 0.37  3.00 0.48 0.30 0.31 0.28 0.24 0.48  4.000.57 0.36 0.34 0.31 0.25 0.57  5.00 0.67 0.41 0.35 0.33 0.29 0.69  6.000.72 0.41 0.37 0.34 0.29 0.73  7.00 0.77 0.45 0.38 0.37 0.33 0.81  8.000.80 0.49 0.41 0.36 0.33 0.85  9.00 0.87 0.48 0.43 0.38 0.34 0.92 10.000.88 0.49 0.44 0.42 0.35 0.93 11.00 0.91 0.48 0.46 0.40 0.35 0.96 12.000.95 0.51 0.46 0.42 0.37 1.00 13.00 0.97 0.52 0.49 0.44 0.37 1.06 14.000.98 0.52 0.48 0.46 0.37 1.03 15.00 0.99 0.52 0.48 0.45 0.37 1.05 16.001.00 0.53 0.49 0.44 0.37 1.09 17.00 1.03 0.55 0.51 0.44 0.35 1.12 18.001.06 0.53 0.50 0.44 0.40 1.11 19.00 1.05 0.56 0.50 0.47 0.39 1.12 20.001.07 0.58 0.52 0.48 0.40 1.14 21.00 1.09 0.57 0.52 0.47 0.40 1.13 22.001.08 0.61 0.51 0.46 0.40 1.14 23.00 1.09 0.57 0.52 0.47 0.41 1.15 24.001.12 0.58 0.52 0.47 0.41 1.17 25.00 1.11 0.56 0.53 0.49 0.41 1.13

Table 11 represents a replicate trial of the experimental conditions inTable 9 and results in Table 10, except that instead of control DatasetID “A6” where the foam was allowed to dry for one week at the conclusionof the experiment, in Table 11 there was control Dataset ID “A8” whichwas a repeat of the dry state after exposure of the foam to the processemulating the addition of water but without a water drop present on thealuminum plate. Recorded thermal power in the Table 11 datasets wasbetween 7.8 and 8.2 milliwatts. Results are contained in Table 12, withprocessing as described for Table 10.

TABLE 11 Measured Water Bottom Mass Layer per Equivalent Foam WaterSurface minutes at Water % of Dataset Mass Mass Area 0.7 mg/ Foam 1.5cm³ ID (g) (g) (mg/cm²) cm²/min Volume A7 0.20 — — — — A8 0.20 0.00 0 0 0% A9 0.27 0.07 11 16  5% A10 0.35 0.15 24 34 10% A11 0.51 0.31 50 7121% A12 0.81 0.61 97 138 40%

TABLE 12 ΔT per average power (° C./mW) t (s) A7 A8 A9 A10 A11 A12  0.000.00 0.00 0.00 0.00 0.00 0.00  1.00 0.21 0.19 0.18 0.13 0.14 0.14  2.000.38 0.34 0.27 0.23 0.19 0.20  3.00 0.48 0.49 0.31 0.28 0.22 0.25  4.000.58 0.56 0.35 0.34 0.29 0.27  5.00 0.67 0.64 0.41 0.36 0.34 0.30  6.000.73 0.70 0.44 0.38 0.35 0.32  7.00 0.78 0.76 0.44 0.42 0.33 0.33  8.000.83 0.78 0.48 0.40 0.37 0.36  9.00 0.89 0.82 0.49 0.44 0.39 0.34 10.000.90 0.88 0.50 0.46 0.39 0.37 11.00 0.96 0.90 0.54 0.47 0.40 0.37 12.000.96 0.93 0.54 0.48 0.41 0.40 13.00 0.99 0.98 0.53 0.48 0.45 0.43 14.001.00 0.93 0.56 0.50 0.46 0.40 15.00 1.00 0.97 0.57 0.47 0.43 0.42 16.001.04 1.00 0.59 0.50 0.44 0.42 17.00 1.06 1.04 0.57 0.49 0.44 0.42 18.001.06 1.01 0.56 0.52 0.47 0.45 19.00 1.11 1.07 0.62 0.52 0.50 0.44 20.001.09 1.06 0.56 0.51 0.50 0.45 21.00 1.11 1.06 0.62 0.55 0.49 0.45 22.001.11 1.07 0.62 0.53 0.48 0.45 23.00 1.12 1.12 0.61 0.53 0.49 0.46 24.001.11 1.12 0.61 0.53 0.50 0.44 25.00 1.15 1.08 0.62 0.56 0.50 0.44

EXEMPLARY EMBODIMENTS

Embodiment A1. A wireless sensing device in an assembly for measuringthermal property of an object, comprising: a thermal spreader comprisingsolid or liquid material, the thermal spreader having a first majorsurface and a second major surface opposite to the first major surface,the thermal spreader configured to be in thermal contact with the objectwhen the wireless sensing device is in use; a control circuit; awireless transceiver electronically coupled to the control circuit; anenergy harvesting device; a thermal source disposed proximate to thesecond major surface of the thermal spreader, the thermal sourceelectronically coupled to the energy harvesting device and configured togenerate a thermal flux to the first major surface of the thermalspreader, wherein the energy harvesting device provides power to thethermal source; and a sensor electronically coupled to the controlcircuit and in thermal contact with the thermal source, wherein thesensor is configured to generate a sensor signal associated withtemperature and provide the sensor signal to the control circuit.

Embodiment A2. The wireless sensing device of Embodiment A1, furthercomprising: an antenna electronically coupled to the transceiver and theenergy harvesting device.

Embodiment A3. The wireless sensing device of Embodiment A2, furthercomprising: a substrate, wherein the antenna is disposed on thesubstrate.

Embodiment A4. The wireless sensing device of Embodiment A3, wherein theantenna is configured to receive a first power when a readerinterrogates the wireless sensing device and the energy harvestingdevice is configured to convert the first power to a second power.

Embodiment A5. The wireless sensing device of Embodiment A1-A4, furthercomprising:

a coupling device configured to maintain thermal contact between thethermal spreader and the object.

Embodiment A6. The wireless sensing device of Embodiment A5, wherein thecoupling device comprises at least one of a thermally conductiveadhesive layer, elastic coupler, and mechanical coupler.

Embodiment A7. The wireless sensing device of Embodiment A1-A6, whereinthe thermal source comprises at least one component of the controlcircuit.

Embodiment A8. The wireless sensing device of Embodiment A1, wherein thethermal source and the sensor are a same resistive element.

Embodiment A9. The wireless sensing device of any one of EmbodimentA1-A8, wherein the control circuit comprises a microprocessor and amemory storing a unique identifier.

Embodiment A10. The wireless sensing device of Embodiment A1-A9, whereinthe energy harvesting device comprise at least one of a bridgerectifier, a diode rectifier, a transistor rectifier, a voltageregulator and a current regulator.

Embodiment A11. The wireless sensing device of Embodiment A1-A10,wherein the wireless sensing device regulates power provided the thermalsource.

Embodiment A12. The wireless sensing device of Embodiment A11, whereinthe wireless sensing device regulates the thermal source based on thesensor signal.

Embodiment A13. The wireless sensing device of Embodiment A12, whereinthe wireless sensing device deactivates the thermal source based on thesensor signal.

Embodiment A14. The wireless sensing device of Embodiment A1-A13,wherein the sensor is configured to generate a first sensor signalbefore the thermal source is activated and a second sensor signal afterthe thermal source is activated.

Embodiment A15. The wireless sensing device of Embodiment A13, whereinthe control circuit is configured to determine a thermal property of theobject based on the first and second sensor signals.

Embodiment A16. The wireless sensing device of Embodiment A15, whereinthe control circuit provides a generally constant power to the thermalsource with a known power magnitude, wherein the control circuitdetermines a thermal property of the object based on the first sensorsignal, the second sensor signal, and the know power magnitude.

Embodiment A17. The wireless sensing device of Embodiment A2, whereinthe control circuit comprises an integrated capacitance, wherein thewireless sensing device receives power when a reader interrogates thewireless sensing device, and wherein the control circuit modifies theintegrated capacitance based on the sensor signal and/or the receivedpower.

Embodiment A18. A wireless sensing device in an assembly for measuring athermal property of an object, comprising: an antenna and a transceiverconfigured to receive a first power wirelessly; an energy harvestingdevice configured to transform the first power to a second power; athermal source electronically couple to the energy harvesting device,wherein the energy harvesting device provides the second power to thethermal source; a control circuit comprising a microprocessor; and asensor electronically coupled to the control circuit and thermallycoupled to the thermal source, wherein the sensor is configured togenerate a first sensor signal before the thermal source is activatedand a second sensor signal after the thermal source is activated, andwherein the control circuit is configured to determine a thermalproperty of the object based on the first and second sensor signals.

Embodiment A19. The wireless sensing device of Embodiment A18, furthercomprising: a substrate, and a thermal spreader comprising solid orliquid material, the thermal spreader configured to be in thermalcontact with the object, wherein the thermal source and the sensor aredisposed in the thermal spreader.

Embodiment A20. The wireless sensing device of Embodiment A18-A19,wherein the control circuit regulates an output of the thermal source.

Embodiment A21. The wireless sensing device of Embodiment A20, whereinthe control circuit regulates the thermal source based on the secondsensor signal generated by the sensor.

Embodiment A22. The wireless sensing device of Embodiment A18-A21,wherein the thermal source and the sensor are a same resistive element.

Embodiment A23. The wireless sensing device of Embodiment A18-A22,wherein the control circuit comprises a microprocessor and a memorystoring a unique identifier.

Embodiment A24. The wireless sensing device of Embodiment A18-A23,wherein the energy harvesting device comprises a bridge rectifier, adiode rectifier, a transistor rectifier, a voltage regulator and acurrent regulator.

Embodiment B1. A wireless sensing device configured to measure aphysical property of an object, comprising: a substrate; an antennadisposed on the substrate; a first control circuit electronicallycoupled to the antenna, the first control circuit comprising a firstmemory storing a first unique identifier and a first transceiver; asecond control circuit electronically coupled to the antenna, the secondcontrol circuit comprising a second memory storing a second uniqueidentifier and a second transceiver; a first excitation deviceconfigured to generate a first excitation signal to change a physicalproperty of the object; a first sensor electronically coupled to thefirst control circuit, wherein the first sensor is configured togenerate a first sensor signal associated with the physical property;and a second sensor electronically coupled to the second controlcircuit, wherein the second sensor is configured to generate a secondsensor signal.

Embodiment B2. The wireless sensing device of Embodiment B1, wherein thefirst excitation device comprises at least one of thermal excitationdevice, light excitation device, sound excitation device, vibrator,voltage source, current source, and electromagnet.

Embodiment B3. The wireless sensing device of Embodiment B2, wherein thefirst sensor or the second sensor comprises at least one of thermalsensor, photodiode, microphone, accelerometer, voltage sensor, currentsensor, and magnetometer.

Embodiment B4. The wireless sensing device of Embodiment B1-B3, furthercomprising: a first sensing region, wherein the first excitation deviceand the first sensor are disposed in the first sensing region.

Embodiment B5. The wireless sensing device of Embodiment B4, furthercomprising: a second sensing region, wherein the second sensor aredisposed in the second sensing region.

Embodiment B6. The wireless sensing device of Embodiment B5, wherein thefirst sensor and the second sensor are thermal sensors, and wherein thefirst sensing region and the second region are thermally isolated.

Embodiment B7. The wireless sensing device of Embodiment B1-B6, furthercomprising:

a second excitation device configured to generate a second excitationsignal.

Embodiment B8. The wireless sensing device of Embodiment B1-B7, whereinthe antenna is configured to provide power to the wireless sensingdevice when a reader interrogates.

Embodiment B9. The wireless sensing device of Embodiment B1-B8, furthercomprising:

an energy harvesting device electronically coupled to the antenna andconfigured to provide power to the first sensor and the second sensor.

Embodiment B10. The wireless sensing device of Embodiment B9, whereinthe energy harvesting device comprises at least one of a bridgerectifier, a diode rectifier, a transistor rectifier, a voltageregulator and a current regulator.

Embodiment B11. The wireless sensing device of Embodiment B1-B10,further comprising:

a processor electronically coupled to the first control circuit and thesecond control circuit, wherein the processor is configured to determinean indicator indicative of the physical property of the object based onthe first sensor signal and the second sensor signal.

Embodiment B12. The wireless sensing device of Embodiment B1-B11,wherein the second sensor signal is associated with the physicalproperty of the object.

Embodiment B13. The wireless sensing device of Embodiment B1-B12,wherein the first control circuit regulates the first excitation device.

Embodiment B14. The wireless sensing device of Embodiment B13, whereinthe first control circuit regulates the first excitation device usingthe first sensor signal.

Embodiment B15. A wireless sensing device configured to measure athermal property of an object, comprising: a first thermal spreader anda second thermal spreader being thermally insulated from the firstthermal spreader; an RF circuit and an antenna electronically coupled tothe RF circuit; an energy harvesting device; a first thermal sourcedisposed in the first thermal spreader and electronically coupled to theenergy harvesting device; a first sensor thermally coupled to the firstthermal source, wherein the first sensor is configured to generate afirst sensor signal associated with temperature; a second thermal sourcedisposed in the second thermal spreader and electronically coupled tothe energy harvesting device, and a second sensor thermally coupled tothe second thermal source, wherein the second sensor is configured togenerate a second sensor signal associated with temperature, wherein theenergy harvesting device is configured to provide a first power to thefirst thermal source and a second power to the second thermal source,wherein the first power has a known ratio to the second power.

Embodiment B16. The wireless sensing device of Embodiment B15, whereinthe second thermal source is in thermal contact with the object.

Embodiment B17. The wireless sensing device of Embodiment B15-B16,wherein the first thermal source and the first sensor are a sameresistive element.

Embodiment B18. The wireless sensing device of Embodiment B15-B17,wherein the RF circuit comprises a microprocessor and a memory storing aunique identifier.

Embodiment B19. The wireless sensing device of Embodiment B15-B18,wherein the energy harvesting device comprises at least one of a bridgerectifier, a diode rectifier, a transistor rectifier, a voltageregulator and a current regulator.

Embodiment B20. The wireless sensing device of Embodiment B18, whereinthe RF circuit is configured to determine a thermal property of theobject based on the first and second sensor signals.

Embodiment B21. The wireless sensing device of Embodiment B15-B20,wherein the first power is equal to the second power.

Embodiment C1. An RF hydration sensor in an assembly, comprising:

a substrate;

an antenna disposed on the substrate;

an RF circuit electrically coupled to the antenna, the RF circuitcomprising a processor;

a thermal source electrically coupled to the RF circuit for changing athermal condition of a target area; and

a sensing element thermally coupled to the thermal source for sensing atemperature of the thermal source, such that when the thermal source isthermally coupled to the target area, the RF hydration sensor wirelesslyreceives a first power having a first form from a transceiver, the RFcircuit transforms the first power to a second power having a secondform different from the first form and delivers the second power to thethermal source, the sensing element senses a time variation of thethermal source temperature, and the processor determines a hydrationindicator indicating hydration level based on the sensed time variationof the thermal source temperature.

Embodiment C2. The RF hydration sensor of Embodiment C1, furthercomprising: a memory storing a reference data associated with hydrationlevel, and wherein the processor is configured to determine thehydration indicator using the reference data.

Embodiment C3. The RF hydration sensor of Embodiment C1 or C2, whereinthe RF circuit controls a magnitude of the second power.

Embodiment C4. The RF hydration sensor of any one of Embodiment C1 toEmbodiment C3, wherein the RF circuit is configured to adjust durationof power supplied to the thermal source.

Embodiment C5. The RF hydration sensor of any one of Embodiment C1 toEmbodiment C4, further comprising: a thermal spreader comprising a solidor liquid material, wherein the thermal source is disposed proximate tothe thermal spreader.

Embodiment C6. The RF hydration sensor of Embodiment C5, wherein thethermal spreader is adapted to substantially uniformly distributethermal flux from the thermal source across the target area when thethermal spreader is in thermal contact with the target area.

Embodiment C7. The RF hydration sensor of any one of Embodiment C1 toEmbodiment C6, wherein the first form is an AC form and the second formis a DC form.

Embodiment C8. The RF hydration sensor of any one of Embodiment C1 toEmbodiment C7, wherein the RF hydration sensor is configured towirelessly receive an unknown first power having a first form from atransceiver, and wherein the RF circuit transforms the unknown firstpower to a known second power having a second form different from thefirst form.

Embodiment C9. The RF hydration sensor of any one of Embodiment C1 toEmbodiment C8, wherein the RF circuit is configured to change themagnitude of the second power by changing a resonant frequency of the RFhydration sensor.

Embodiment C10. The RF hydration sensor of any one of Embodiment C1 toEmbodiment C9, wherein the processor is configured to determine whethera thermal steady state is reached, and wherein the RF circuitdeactivates the thermal source after the thermal steady state based onthe sensed time variation of the thermal source temperature.

Embodiment C11. The RF hydration sensor of any one of Embodiment C1 toEmbodiment C10, wherein the sensing element senses a cooling timevariation of the thermal source temperature after the thermal source isdeactivated, and wherein the processor determines a thermal diffusivityof the target area based on the sensed cooling time variation of thethermal source temperature, and wherein the processor determines thehydration indicator using the determined thermal diffusivity of thetarget area.

Embodiment C12. The RF hydration sensor of any one of Embodiment C1 toEmbodiment C11, wherein the RF hydration sensor is adapted to wirelesslycommunicate with a remote transceiver emitting power at a first radiofrequency, wherein the RF circuit is adapted to detune a resonantfrequency of the RF hydration sensor away from the first radio frequencyto control a magnitude of the first power received by the RF hydrationsensor from the remote transceiver.

Embodiment C13. The RF hydration sensor of any one of Embodiment C1 toEmbodiment C12, wherein the substrate is flexible.

Embodiment C14. The RF hydration sensor of any one of Embodiment C1 toEmbodiment C13, wherein the substrate is stretchable.

Embodiment C15. An RF sensor for measuring a liquid level, comprising:

a substrate;

an antenna disposed on the substrate;

an RF circuit electrically coupled to the antenna, the RF circuitcomprising a processor;

an absorption element comprising absorption material,

a thermal source electrically coupled to the RF circuit and thermallycoupled to the absorption element; and

a sensing element thermally coupled to the thermal source for sensing atemperature of the thermal source, such that after the absorptionelement is used to absorb liquid, the RF sensor wirelessly receives afirst power having a first form from a transceiver, the RF circuittransforms the first power to a second power having a second formdifferent from the first form and delivers the second power to thethermal source, the sensing element senses a time variation of thethermal source temperature, and the processor determines an indicatorindicating liquid level based on the sensed time variation of thethermal source temperature.

Embodiment C16. The RF sensor of Embodiment C15, wherein absorptionmaterial comprises at least one of a porous material, a natural orsynthetic sponge, water-absorbing gel, and superabsorbent polymer.

Embodiment C17. The RF sensor of Embodiment C15 or 16, wherein the RFcircuit controls a magnitude of the second power.

Embodiment C18. The RF sensor of any one of Embodiment C15 to EmbodimentC17, wherein the RF circuit is configured to adjust duration of powersupplied to the thermal source.

Embodiment C19. The RF sensor of any one of Embodiment C15 to EmbodimentC18, wherein the first form is an AC form and the second form is a DCform.

Embodiment C20. A method of determining hydration level using one ormore processors and a sensor having a thermal source disposed proximateto an object, comprising:

wirelessly activating the thermal source;

generating a series of sensing signals by the sensor;

determining, by the one or more processors, a thermal property of theobject based on at least some of the series of sensing signals; and

generating, by the one or more processors, a hydration indicatorindicative of hydration level of the object based on the determinedthermal property and a reference data.

Embodiment C21. The method of Embodiment C20, further comprising:

generating a first sensing signal;

evaluating, by the one or more processors, whether a thermal steadystate is reached;

wherein the determining step comprises determining a thermalconductivity of the object based on the first sensing signal and atleast one of the series of sensing signals generated when the thermalsteady state is reached,

and wherein the hydration indicator is generated based on the determinedthermal conductivity.

Embodiment C22. The method of Embodiment C20 or 21, further comprising:

evaluating, by the one or more processors, whether a thermal steadystate is reached;

deactivating the thermal source after the thermal steady state isreached.

Embodiment C23. The method of any one of Embodiment C20 to EmbodimentC22, further comprising:

generating a series of cooling sensing signals after the thermal sourceis deactivated;

wherein the determining step comprises determining a thermal diffusivityof the object based on at least some of the series of cooling sensingsignals,

and wherein the hydration indicator is generated based on the determinedthermal diffusivity.

Embodiment C24. The method of any one of Embodiment C20 to EmbodimentC23, wherein the reference data comprises at least one of an analyticalfunction, a look-up table, a matrix, and a constant.

Embodiment C25. The method of any one of Embodiment C20 to EmbodimentC24, further comprising: generating a calibration signal by the sensorwhen the sensor is disposed to a reference material with a known thermalproperty, wherein the determining step comprises determining the thermalproperty of the object using the calibration signal.

Embodiment C26. An RF hydration sensing system, comprising:

an RF sensor tag, comprising:

a substrate;

an antenna disposed on the substrate;

an RF circuit electrically coupled to the antenna;

a thermal source electrically coupled to the RF circuit for changing athermal condition of a target area; and

a sensing element thermally coupled to the thermal source for sensing atemperature of the thermal source, such that when the thermal source isthermally coupled to a target area, the RF sensor tag wirelesslyreceives a first power having a first form from a transceiver, the RFcircuit transforms the first power to a second power having a secondform and delivers the second power to the thermal source, the sensingelement senses a time variation of the thermal source temperature, andthe RF sensor tag wirelessly transmits the sensed time variation of thethermal source temperature,

an RF reader configured to wirelessly transmit an interrogation powerthe RF sensor tag and receive the sensed time variation of the thermalsource temperature,

a processor electronically coupled to the RF reader and configured todetermine a hydration indicator indicative of hydration level based onthe sensed time variation of the thermal source temperature.

Embodiment C27. The RF hydration sensing system of Embodiment C26,further comprising: a memory storing a reference data associated withhydration level, and wherein the processor is configured to determinethe hydration indicator using the reference data.

Embodiment C28. The RF hydration sensing system of Embodiment C26 or 27,wherein the RF circuit controls a magnitude of the second power.

Embodiment C29. The RF hydration sensing system of any one of EmbodimentC26 to Embodiment C28, wherein the RF circuit is configured to adjustduration of power supplied to the thermal source.

Embodiment C30. The RF hydration sensing system of any one of EmbodimentC26 to Embodiment C29, wherein the RF sensor tags further comprises: athermal spreader comprising a solid or liquid material, wherein thethermal source is disposed proximate to the thermal spreader.

Embodiment C31. The RF hydration sensing system of Embodiment C30,wherein the thermal spreader is adapted to substantially uniformlydistribute thermal flux from the thermal source across the target areawhen the thermal spreader is in thermal contact with the target area.

Embodiment C32. The RF hydration sensing system of any one of EmbodimentC26 to Embodiment C31, wherein the first form is an AC form and thesecond form is a DC form.

Embodiment C33. The RF hydration sensing system of any one of EmbodimentC26 to Embodiment C32, wherein the RF sensor tag is configured towirelessly receive an unknown first power having a first form from atransceiver, and wherein the RF circuit transforms the unknown firstpower to a known second power having a second form different from thefirst form.

Embodiment C34. The RF hydration sensing system of any one of EmbodimentC26 to Embodiment C33, wherein the RF reader is configured to change theinterrogation power based on sensed time variation of the thermal sourcetemperature.

Embodiment C35. The RF hydration sensing system of any one of EmbodimentC26 to Embodiment C34, further comprising: a coupling device configuredto maintain thermal contact between the RF sensor tag and the targetarea.

Embodiment C36. The RF hydration sensing system of Embodiment C35,wherein the coupling device comprises at least one of a thermallyconductive adhesive layer, an adhesive layer, an elastic coupler, and amechanical coupler.

Embodiment D1. A mobile sensing system, comprising: a sensing device formeasuring a thermal property of an object, comprising: an RF circuit andan antenna electronically coupled to the RF circuit, a sensorelectronically coupled to the RF circuit, and a thermal source thermallycoupled to the sensor and electronically coupled to the RF circuit, amobile device having a processor, an RF reader connected to orintegrated with the mobile device, wherein the RF reader is configuredto interrogate the sensing device; wherein the sensing device receivespower when the RF reader interrogates the sensing device and provides atleast a portion of the power to the thermal source, wherein the sensoris configured to generate a series of sensing signals associated withtemperature when the thermal source is powered, wherein the sensingdevice is configured to transmit a series of data signals associatedwith the series of sensing signals via the antenna; and wherein the RFreader receives the series of data signals and the processor isconfigured to determine the thermal property of the object using atleast some of the series of data signals.

Embodiment D2. The mobile sensing system of Embodiment D1, wherein thesensing device further comprises a substrate, wherein the RF circuit andthe antenna are disposed on the substrate.

Embodiment D3. The mobile sensing system of Embodiment D1 or EmbodimentD2, further comprising: a thermal spreader having a first major surfaceand a second opposing major surface, such that when the sensing deviceis in use, the thermal spreader is in thermal contact with the objectand the first major surface is proximate to the object, and the thermalsource is disposed proximate to the second major surface, the thermalsource generates a substantially uniform thermal flux on the first majorsurface of the thermal spreader.

Embodiment D4. The mobile sensing system of Embodiment D3, wherein thegenerated thermal flux is modified by the object when the sensing deviceis in use.

Embodiment D5. The mobile sensing system of Embodiment D3, wherein thethermal spreader has a known thermal property, wherein the processor isfurther configured to determine the thermal property of the object usingthe known thermal property of the thermal spreader.

Embodiment D6. The mobile sensing system of any one of Embodiment D1 toEmbodiment D5, wherein the sensing device further comprises: an energyharvesting device electronically coupled to the RF circuit andconfigured to convert the power received by the sensing device to powerin DC form.

Embodiment D7. The mobile sensing system of Embodiment D6, wherein theenergy harvesting device comprises at least one of a bridge rectifier, adiode rectifier, a transistor rectifier, a voltage regulator and acurrent regulator.

Embodiment D8. The mobile sensing system of any one of Embodiment D1 toEmbodiment D7, wherein the RF reader is configured to adjustinterrogation power based on at least one of the series of data signals.

Embodiment D9. The mobile sensing system of any one of Embodiment D1 toEmbodiment D8, wherein the sensing device is configured to adjust powersupplied to the thermal source.

Embodiment D10. The mobile sensing system of Embodiment D9, wherein thesensing device is configured to adjust power supplied to the thermalsource based on one of the series of sensing signals.

Embodiment D11. The mobile sensing system of Embodiment D10, wherein thesensing device is configured to deactivate the thermal source based onat least one of the series of sensing signals.

Embodiment D12. A wireless sensing system, comprising: an RF sensor tagconfigured to evaluate a physical characteristic of an object,comprising: an RF circuit and an antenna electronically coupled to theRF circuit, a sensor electronically coupled to the RF circuit, and anexcitation device configured to be activated and generate an excitationsignal to change a condition, wherein the sensor generates a sensingsignal associated with the physical characteristic of the object thatvaries in response to the changed condition when the excitation deviceis activated, wherein the RF sensor tag is configured to transmit a datasignal associated with the sensing signal via the antenna; and an RFreader configured to transmit an interrogation power to the RF sensortag to activate the excitation device and receive the data signal,wherein the RF reader is further configured to adjust the interrogationpower based on the data signal.

Embodiment D13. The wireless sensing system of Embodiment D12, furthercomprising: a computing device, wherein the reader is connected to orintegrated with the computing device.

Embodiment D14. The wireless sensing system of Embodiment D13, whereinthe computing device determines the physical characteristic of theobject based on the data signal.

Embodiment D15. The wireless sensing system of any one of Embodiment D12to Embodiment D14, wherein the RF sensor tag comprises a plurality ofsensors.

Embodiment D16. The wireless sensing system of Embodiment D15, whereinone of the plurality of sensors provides a reference sensor signalindependent from the physical characteristic of the object.

Embodiment D17. The wireless sensing system of any one of Embodiment D12to Embodiment D16, wherein the excitation device comprises at least oneof thermal excitation device, light excitation device, sound excitationdevice, vibrator, voltage source, current source, and electromagnet.

Embodiment D18. The wireless sensing system of any one of Embodiment D12to Embodiment D17, wherein the sensor comprises at least one of thermalsensor, photodiode, photo sensor, microphone, accelerometer, voltagesensor, current sensor, and magnetometer.

Embodiment D19. The wireless sensing system of any one of Embodiment D12to Embodiment D18, further comprising: a sensing region comprisingmaterials suitable to propagate the excitation signal, wherein theexcitation device is disposed proximate to the sensing region.

Embodiment D20. The wireless sensing system of any one of Embodiment D12to Embodiment D19, wherein the RF reader is further configured to reducethe interrogation power based on the data signal.

Embodiment D21. The wireless sensing system of any one of Embodiment D12to Embodiment D20, wherein the RF sensor tag comprises a plurality ofexcitation devices and provide power to the plurality of excitationdevices, wherein the power provided to each of the plurality ofexcitation devices has a known ratio to power received by the RF sensortag when the RF reader interrogates.

Embodiment D22. A wireless sensing system, comprising: a plurality ofsensing devices, each sensing device comprising: a wireless transceiverand an antenna electronically coupled to the wireless transceiver, asensor electronically coupled to the wireless transceiver, and anexcitation device configured to generate an excitation signal, whereinthe sensor produces a sensing signal when the excitation signal isgenerated, wherein the sensing device is configured to transmit a datasignal associated with the sensing signal via the antenna; and a readerconfigured to transmit activation signals to the plurality of sensingdevices to activate the excitation devices and receive data signals fromthe plurality of sensing devices, wherein at least some of the pluralityof sensing devices are disposed at locations different from each other,wherein the reader is further configured to coordinate a transmissionorder of activation signals to the plurality of sensing devices in apattern related to the location of sensing devices.

Embodiment D23. The wireless sensing system of Embodiment D22, furthercomprising: a computing device, wherein the reader is connected to orintegrated with the computing device.

Embodiment D24. The wireless sensing system of Embodiment D23, whereinat least some of the plurality of sensing devices are disposed proximatean object, and wherein the computing device is configured to determine aphysical characteristic of the object based on the data signals.

Embodiment D25. The wireless sensing system of Embodiment D24, whereinat least one of the plurality of sensing devices is configured totransmit a reference data signal correspond to a reference sensor signalindependent from the physical characteristic of the object, and whereinthe computing device is further configured to determine the physicalcharacteristic of the object using the reference data signal.

Embodiment D26. The wireless sensing system of any one of Embodiment D22to Embodiment D25, wherein at least one of the plurality of sensingdevices is configured to transmit a power signal indicating power levelof the sensing device, and wherein the reader is further configured tocoordinate the transmission order of activation signals based on thepower signal.

The present invention should not be considered limited to the particularexamples and embodiments described above, as such embodiments aredescribed in detail to facilitate explanation of various aspects of theinvention. Rather the present invention should be understood to coverall aspects of the invention, including various modifications,equivalent processes, and alternative devices falling within the spiritand scope of the invention as defined by the appended claims and theirequivalents.

What is claimed is:
 1. A wireless sensing system comprising: a radiofrequency (RF) sensor tag configured to evaluate a physicalcharacteristic of an object, the RF sensor tag comprising: a stretchablesubstrate; an RF circuit; an antenna electronically coupled to the RFcircuit and disposed on the stretchable substrate; a sensorelectronically coupled to the RF circuit; and a current sourceconfigured to be activated and generate a current signal to the object,wherein the sensor generates a sensing signal associated with thephysical characteristic of the object that varies in response to achange in an electrical property of the object when the current signalis received at the object, wherein the RF sensor tag is configured totransmit a data signal associated with the sensing signal via theantenna; and an RF reader configured to transmit an interrogation powerto the RF sensor tag to activate the current source and receive the datasignal.
 2. The wireless sensing system of claim 1, wherein the RF readeris further configured to adjust the interrogation power based on thedata signal.
 3. The wireless sensing system of claim 2, wherein thecurrent signal changes the electrical property of the object, andwherein the physical characteristic of the object varies in response tothe changed electrical property of the object.
 4. The wireless sensingsystem of claim 1, further comprising a computing device, wherein the RFreader is connected to the computing device.
 5. The wireless sensingsystem of claim 1, wherein the RF sensor tag comprises a plurality ofsensors.
 6. The wireless sensing system of claim 1, further comprising asensing region comprising materials suitable to propagate the currentsignal, wherein the current source is disposed proximate to the sensingregion.
 7. A wireless sensing system comprising: a radio frequency (RF)sensor tag configured to evaluate a physical characteristic of anobject, the RF sensor tag comprising: a stretchable substrate; an RFcircuit; an antenna electronically coupled to the RF circuit anddisposed on the stretchable substrate; a sensor electronically coupledto the RF circuit; and an optical source configured to be activated andgenerate a light signal to the object, wherein the sensor generates asensing signal associated with the physical characteristic of the objectthat varies in response to a condition of the object when the lightsignal is received at the object, wherein the RF sensor tag isconfigured to transmit a data signal associated with the sensing signalvia the antenna; and an RF reader configured to transmit aninterrogation power to the RF sensor tag to activate the optical sourceand receive the data signal.
 8. The wireless sensing system of claim 7,wherein the RF reader is further configured to adjust the interrogationpower based on the data signal.
 9. The wireless sensing system of claim8, wherein the light signal changes the condition of the object, andwherein the physical characteristic of the object varies in response tothe changed condition of the object.
 10. The wireless sensing system ofclaim 7, wherein the condition of the object is associated with anoptical property of the object.
 11. The wireless sensing system of claim7, further comprising a computing device, wherein the RF reader isconnected to the computing device.
 12. The wireless sensing system ofclaim 7, wherein the RF sensor tag comprises a plurality of sensors. 13.A wireless sensing system comprising: a radio frequency (RF) sensor tagconfigured to evaluate a physical characteristic of an object, the RFsensor tag comprising: a stretchable substrate; an RF circuit; anantenna electronically coupled to the RF circuit and disposed on thestretchable substrate; a sensor electronically coupled to the RFcircuit; and a motion source configured to be activated and generate amotion-based excitation signal to the object, wherein the sensorgenerates a sensing signal associated with the physical characteristicof the object that varies in response to a condition of the object whenthe motion-based excitation signal is received at the object, whereinthe RF sensor tag is configured to transmit a data signal associatedwith the sensing signal via the antenna; and an RF reader configured totransmit an interrogation power to the RF sensor tag to activate themotion source and receive the data signal.
 14. The wireless sensingsystem of claim 13, wherein the RF reader is further configured toadjust the interrogation power based on the data signal.
 15. Thewireless sensing system of claim 14, wherein the motion-based excitationsignal changes the condition of the object, and wherein the physicalcharacteristic of the object varies in response to the changed conditionof the object.
 16. The wireless sensing system of claim 13, furthercomprising a computing device, wherein the RF reader is connected to thecomputing device.
 17. The wireless sensing system of claim 13, whereinthe RF sensor tag comprises a plurality of sensors.
 18. The wirelesssensing system of claim 13, further comprising a sensing regioncomprising materials suitable to propagate the motion-based excitationsignal, wherein the motion source is disposed proximate to the sensingregion.
 19. A wireless sensing system comprising: a radio frequency (RF)sensor tag configured to evaluate a physical characteristic of anobject, the RF sensor tag comprising: a stretchable substrate; an RFcircuit; an antenna electronically coupled to the RF circuit anddisposed on the stretchable substrate; a sensor electronically coupledto the RF circuit; and a voltage source configured to be activated andgenerate an electrical signal that changes in conduction through theobject based on an electrical property of the object, wherein the sensorgenerates a sensing signal associated with the electrical property inresponse to the conduction of the electrical signal when the electricalsignal is conducted through the object, wherein the RF sensor tag isconfigured to transmit a data signal associated with the sensing signalvia the antenna; and an RF reader configured to transmit aninterrogation power to the RF sensor tag to activate the voltage sourceand receive the data signal.